Add password-encrypted settings export/import

Closes #35. Exports the host environment — global AppSettings (already
the non-secret shape persisted to settings.json) plus the global secrets
that live in the OS keychain instead (the shared Claude Code OAuth login,
the model gateway's provider API key and master key) — to one
password-encrypted file, and restores it on another machine.
Per-project settings, per-project secrets, and Docker volumes are
deliberately out of scope; this is not a project backup.

Designed with the user in issue #35's comments: global settings only, no
docker volumes, the password is the lock/key, and the export is portable
as one file.

Crypto (storage/settings_crypto.rs): Argon2id derives a 256-bit key from
the password (memory-hard, meaningfully resistant to GPU/ASIC
brute-forcing in a way PBKDF2 at any reasonable iteration count is not),
AES-256-GCM does the actual encryption. A wrong password fails GCM's
authentication tag rather than producing silent garbage. Salt and nonce
are random per export and stored in the clear in the file header — their
job is uniqueness, not secrecy.

The save/open dialogs are opened from Rust, matching the boundary
file_commands.rs's pick_save_path/pick_files_to_upload already establish:
a frontend-driven dialog handing Rust a host path is the exact shape of
bug that produced this app's past criticals. preview_settings_import
resolves the chosen import path itself and remembers it
(AppState::pending_settings_import) so apply_settings_import re-reads the
same file without a path crossing back over IPC. The password is
re-entered rather than cached between preview and apply, so nothing here
holds decrypted plaintext in memory for longer than one command's
execution; the preview returned to the frontend carries counts and
presence flags only, never a secret value.

Import replaces settings wholesale (an import is "restore this
environment"), but only writes secrets actually present in the file — an
absent secret means "the source machine never had this configured," not
"delete this on import."

Added storage::secure::store_gateway_master_key and get_gateway_master_key
(read-only, unlike get_or_create_gateway_master_key which mints one as a
side effect) since neither existed and import needs to restore an exact
captured value rather than mint a new random one.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FGjXq6fqtAFHdbhk4f3PfZ

app/src-tauri/src/models/settings_export.rs

@@ -0,0 +1,180 @@
+//! Settings export/import — see triple-c#35.
+//!
+//! `SettingsExportPayload` is the whole plaintext export before encryption
+//! and after decryption (see `storage::settings_crypto`). It bundles
+//! `AppSettings` (already the non-secret shape persisted to `settings.json`)
+//! with the global secrets that live in the OS keychain instead — the shared
+//! Claude Code OAuth login and the model gateway's two keys. Per-project
+//! settings, per-project secrets, and anything living in a project's Docker
+//! volumes are deliberately out of scope: this exports the *host*
+//! environment, not any one project's.
+
+use serde::{Deserialize, Serialize};
+
+use super::AppSettings;
+
+/// Bumped when the shape of [`SettingsExportPayload`] changes in a way that
+/// isn't just an additive, `#[serde(default)]`-covered field — e.g. if a
+/// field is ever removed or its meaning changes. `apply_settings_import`
+/// checks this before touching anything.
+pub const SETTINGS_EXPORT_FORMAT_VERSION: u32 = 1;
+
+/// The global secrets bundled into an export. Deliberately a separate struct
+/// from `AppSettings`: these live in the OS keychain, never in
+/// `settings.json`, and — outside of this export/import flow — the values
+/// themselves never cross into the frontend; see the doc comments on
+/// `storage::secure::get_gateway_api_key` and
+/// `commands::settings_export_commands` for why that boundary matters here
+/// too.
+#[derive(Debug, Clone, Serialize, Deserialize, Default)]
+pub struct ExportedSecrets {
+    #[serde(default)]
+    pub claude_oauth_token: Option<String>,
+    #[serde(default)]
+    pub gateway_api_key: Option<String>,
+    #[serde(default)]
+    pub gateway_master_key: Option<String>,
+}
+
+impl ExportedSecrets {
+    pub fn is_empty(&self) -> bool {
+        self.claude_oauth_token.is_none()
+            && self.gateway_api_key.is_none()
+            && self.gateway_master_key.is_none()
+    }
+}
+
+/// The full plaintext payload — this is what gets encrypted on export and
+/// what decryption recovers on import. Never written to disk unencrypted;
+/// see `storage::settings_crypto`.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SettingsExportPayload {
+    pub format_version: u32,
+    /// RFC3339. Purely informational — shown in the import preview so a user
+    /// picking between a few old export files has something to go on.
+    pub exported_at: String,
+    /// The exporting app's `CARGO_PKG_VERSION`. Also informational: every
+    /// field below already round-trips through `#[serde(default)]`-covered
+    /// `AppSettings`, so an older or newer export still deserializes; this is
+    /// for a human to notice "this is from a much older version" if an import
+    /// ever looks wrong, not something the code branches on.
+    pub app_version: String,
+    pub settings: AppSettings,
+    #[serde(default)]
+    pub secrets: ExportedSecrets,
+}
+
+/// What `preview_settings_import` hands the frontend before anything is
+/// applied — counts and presence flags only, **never** a secret value itself,
+/// so this type is safe to return across the IPC boundary and render
+/// directly. The confirmation UI is built from this.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct SettingsImportPreview {
+    pub exported_at: String,
+    pub app_version: String,
+    pub custom_env_var_count: usize,
+    pub gateway_model_count: usize,
+    pub has_claude_code_settings: bool,
+    pub has_claude_oauth_token: bool,
+    pub has_gateway_api_key: bool,
+    pub has_gateway_master_key: bool,
+}
+
+impl SettingsImportPreview {
+    pub fn from_payload(payload: &SettingsExportPayload) -> Self {
+        Self {
+            exported_at: payload.exported_at.clone(),
+            app_version: payload.app_version.clone(),
+            custom_env_var_count: payload.settings.global_custom_env_vars.len(),
+            gateway_model_count: payload.settings.gateway.models.len(),
+            has_claude_code_settings: payload.settings.global_claude_code_settings.is_some(),
+            has_claude_oauth_token: payload
+                .secrets
+                .claude_oauth_token
+                .as_deref()
+                .is_some_and(|t| !t.trim().is_empty()),
+            has_gateway_api_key: payload
+                .secrets
+                .gateway_api_key
+                .as_deref()
+                .is_some_and(|k| !k.trim().is_empty()),
+            has_gateway_master_key: payload
+                .secrets
+                .gateway_master_key
+                .as_deref()
+                .is_some_and(|k| !k.trim().is_empty()),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::models::AppSettings;
+
+    fn payload_with(secrets: ExportedSecrets) -> SettingsExportPayload {
+        let mut settings = AppSettings::default();
+        settings.global_custom_env_vars = vec![
+            crate::models::EnvVar { key: "A".to_string(), value: "1".to_string() },
+            crate::models::EnvVar { key: "B".to_string(), value: "2".to_string() },
+        ];
+        SettingsExportPayload {
+            format_version: SETTINGS_EXPORT_FORMAT_VERSION,
+            exported_at: "2026-08-27T00:00:00Z".to_string(),
+            app_version: "0.4.14".to_string(),
+            settings,
+            secrets,
+        }
+    }
+
+    #[test]
+    fn the_preview_never_carries_a_secret_value() {
+        let payload = payload_with(ExportedSecrets {
+            claude_oauth_token: Some("sk-super-secret-token".to_string()),
+            gateway_api_key: Some("sk-another-secret".to_string()),
+            gateway_master_key: Some("sk-triple-c-yet-another".to_string()),
+        });
+        let preview = SettingsImportPreview::from_payload(&payload);
+        let serialized = serde_json::to_string(&preview).unwrap();
+
+        assert!(!serialized.contains("sk-super-secret-token"));
+        assert!(!serialized.contains("sk-another-secret"));
+        assert!(!serialized.contains("sk-triple-c-yet-another"));
+        assert!(preview.has_claude_oauth_token);
+        assert!(preview.has_gateway_api_key);
+        assert!(preview.has_gateway_master_key);
+    }
+
+    #[test]
+    fn a_blank_secret_reads_as_absent_in_the_preview() {
+        // A keychain entry that exists but holds only whitespace must not
+        // read as "present" — same "blank counts as absent" rule the
+        // keychain layer itself applies when storing these.
+        let payload = payload_with(ExportedSecrets {
+            claude_oauth_token: Some("   ".to_string()),
+            gateway_api_key: None,
+            gateway_master_key: None,
+        });
+        let preview = SettingsImportPreview::from_payload(&payload);
+        assert!(!preview.has_claude_oauth_token);
+        assert!(!preview.has_gateway_api_key);
+        assert!(!preview.has_gateway_master_key);
+    }
+
+    #[test]
+    fn counts_reflect_the_real_settings() {
+        let payload = payload_with(ExportedSecrets::default());
+        let preview = SettingsImportPreview::from_payload(&payload);
+        assert_eq!(preview.custom_env_var_count, 2);
+    }
+
+    #[test]
+    fn an_empty_secrets_bundle_reports_itself_as_empty() {
+        assert!(ExportedSecrets::default().is_empty());
+        assert!(!ExportedSecrets {
+            claude_oauth_token: Some("x".to_string()),
+            ..Default::default()
+        }
+        .is_empty());
+    }
+}