Add app update detection and multi-folder project support

Feature 1 - Update Detection: Query Gitea releases API on startup (3s
delay) and every 24h, compare patch versions by platform, show pulsing
"Update" button in TopBar with dialog for release notes/downloads.
Settings: auto-check toggle, manual check, dismiss per-version.

Feature 2 - Multi-Folder Projects: Replace single `path` with
`paths: Vec<ProjectPath>` (host_path + mount_name). Each folder mounts
to `/workspace/{mount_name}`. Auto-migrate old single-path JSON on load.
Container recreation via paths-fingerprint label. AddProjectDialog and
ProjectCard support add/remove/edit of multiple folders.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -0,0 +1,37 @@
+use serde::{Deserialize, Serialize};
+
+/// Info returned to the frontend about an available update.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct UpdateInfo {
+    pub version: String,
+    pub tag_name: String,
+    pub release_url: String,
+    pub body: String,
+    pub assets: Vec<ReleaseAsset>,
+    pub published_at: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ReleaseAsset {
+    pub name: String,
+    pub browser_download_url: String,
+    pub size: u64,
+}
+
+/// Gitea API release response (internal).
+#[derive(Debug, Clone, Deserialize)]
+pub struct GiteaRelease {
+    pub tag_name: String,
+    pub html_url: String,
+    pub body: String,
+    pub assets: Vec<GiteaAsset>,
+    pub published_at: String,
+}
+
+/// Gitea API asset response (internal).
+#[derive(Debug, Clone, Deserialize)]
+pub struct GiteaAsset {
+    pub name: String,
+    pub browser_download_url: String,
+    pub size: u64,
+}