Add Docker install helper for first-run setup

When Docker isn't detected on startup, surface a dialog offering a
one-click install (pkexec + get.docker.com on Linux, brew cask on
macOS, winget on Windows) with a graceful fallback to manual steps
and a link to official documentation. Install output streams back
to the UI via a tauri event.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/src-tauri/src/install_helper/mod.rs

@@ -0,0 +1,53 @@
+// Helpers for detecting whether Docker (or a Docker-compatible runtime) is
+// installed on the host and, when missing, offering to install it for the user.
+//
+// We use the Docker convenience script on Linux and Rancher Desktop on macOS /
+// Windows. On every platform we also surface an official documentation URL so
+// users without a recognised package manager can install manually.
+
+use serde::{Deserialize, Serialize};
+
+pub mod platform;
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct InstallOptions {
+    /// "linux" | "macos" | "windows" | "unknown"
+    pub os: String,
+    /// User-facing name of what we'd install ("Docker Engine" / "Rancher Desktop").
+    pub product_name: String,
+    /// Whether we can kick off a one-click install with what's on this machine.
+    pub can_auto_install: bool,
+    /// Short identifier of the method we'd use ("pkexec", "brew", "winget", or None).
+    pub auto_install_method: Option<String>,
+    /// If auto-install isn't possible, a human-readable reason to show the user.
+    pub auto_install_blocker: Option<String>,
+    /// Official documentation URL for manual install.
+    pub docs_url: String,
+    /// Ordered manual install steps (plain text lines).
+    pub manual_steps: Vec<String>,
+    /// Notes to display after a successful auto-install (e.g. log out/back in).
+    pub post_install_notes: Vec<String>,
+}
+
+pub fn detect_install_options() -> InstallOptions {
+    if cfg!(target_os = "linux") {
+        platform::linux_options()
+    } else if cfg!(target_os = "macos") {
+        platform::macos_options()
+    } else if cfg!(target_os = "windows") {
+        platform::windows_options()
+    } else {
+        InstallOptions {
+            os: "unknown".into(),
+            product_name: "Docker".into(),
+            can_auto_install: false,
+            auto_install_method: None,
+            auto_install_blocker: Some("Unsupported operating system".into()),
+            docs_url: "https://docs.docker.com/get-docker/".into(),
+            manual_steps: vec![
+                "Visit the Docker documentation and follow the install guide for your OS.".into(),
+            ],
+            post_install_notes: vec![],
+        }
+    }
+}