Close gateway-secret desync, TOCTOU, and undisclosed custom-image gaps

Round 4 review findings:

- Disclose and warn on a custom Docker image the import would set (HIGH):
  it's the image every project container is created from, so an
  undisclosed change here was a sharper version of the redirected-base-URL
  problem round 3 already flagged for the model backends.
- Recreate a running gateway container when an import restores a new
  secret with the shape unchanged (MEDIUM): reconcile_gateway's shape
  comparison can't see a secret-only change, so the container would
  otherwise keep serving old key material indefinitely.
- Report keychain write failures back to the caller instead of only
  logging them (MEDIUM): apply_settings_import now returns
  SettingsImportOutcome with secret_restore_warnings so a partial restore
  can't read as unqualified success.
- Pin a hash of the previewed file's ciphertext and refuse to apply if it
  changed on disk (MEDIUM): closes a TOCTOU between preview and apply.
- Sanitize and cap every free-form string a preview surfaces, and move the
  warning boxes above the replace list in the UI (MEDIUM): an unbounded
  base URL or image name could otherwise push the security warnings below
  the scroll fold.
- Validate the Docker socket path on import the same as the SSH key and CA
  cert paths (LOW): it was the one mounted host path validate_settings_update
  didn't cover.
- Fix ExportedSecrets::is_empty() to treat whitespace-only as blank, like
  every other secret-presence check in this feature (LOW).
- Authenticate the file header as AEAD associated data (LOW, defense in
  depth) and correct two doc comments that overstated the password not
  being cached.

app/src/lib/settingsImportPreview.ts

@@ -25,6 +25,9 @@ export function describeImport(preview: SettingsImportPreview): string[] {
     items.push(`OpenAI-compatible server: ${preview.openai_compatible_base_url}`);
   }
   if (preview.gateway_api_base) items.push(`Gateway upstream: ${preview.gateway_api_base}`);
+  if (preview.image_source === "custom") {
+    items.push(`Docker image: ${preview.custom_image_name ?? "(no image name set)"}`);
+  }
   return items;
 }
 
@@ -41,6 +44,10 @@ export function describeImport(preview: SettingsImportPreview): string[] {
  * would silently become live the next time someone flips the terminal on
  * through the UI, with no import-time signal that it wasn't freshly
  * generated.
+ *
+ * A custom Docker image gets a warning every time, not just on change: it's
+ * the image every project container is created from, so it's worth calling
+ * out regardless of what was configured before the import.
  */
 export function describeImportWarnings(preview: SettingsImportPreview): string[] {
   const warnings: string[] = [];
@@ -51,5 +58,10 @@ export function describeImportWarnings(preview: SettingsImportPreview): string[]
       "Includes a web terminal access token that will activate the next time the web terminal is turned on.",
     );
   }
+  if (preview.image_source === "custom") {
+    warnings.push(
+      `Runs every project container from a custom Docker image: ${preview.custom_image_name ?? "(no image name set)"}.`,
+    );
+  }
   return warnings;
 }