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/hooks/useUpdates.ts

@@ -0,0 +1,72 @@
+import { useCallback, useRef } from "react";
+import { useShallow } from "zustand/react/shallow";
+import { useAppState } from "../store/appState";
+import * as commands from "../lib/tauri-commands";
+
+const CHECK_INTERVAL_MS = 24 * 60 * 60 * 1000; // 24 hours
+
+export function useUpdates() {
+  const { updateInfo, setUpdateInfo, appVersion, setAppVersion, appSettings } =
+    useAppState(
+      useShallow((s) => ({
+        updateInfo: s.updateInfo,
+        setUpdateInfo: s.setUpdateInfo,
+        appVersion: s.appVersion,
+        setAppVersion: s.setAppVersion,
+        appSettings: s.appSettings,
+      })),
+    );
+
+  const intervalRef = useRef<ReturnType<typeof setInterval> | null>(null);
+
+  const loadVersion = useCallback(async () => {
+    try {
+      const version = await commands.getAppVersion();
+      setAppVersion(version);
+    } catch (e) {
+      console.error("Failed to load app version:", e);
+    }
+  }, [setAppVersion]);
+
+  const checkForUpdates = useCallback(async () => {
+    try {
+      const info = await commands.checkForUpdates();
+      if (info) {
+        // Respect dismissed version
+        const dismissed = appSettings?.dismissed_update_version;
+        if (dismissed && dismissed === info.version) {
+          setUpdateInfo(null);
+          return null;
+        }
+      }
+      setUpdateInfo(info);
+      return info;
+    } catch (e) {
+      console.error("Failed to check for updates:", e);
+      return null;
+    }
+  }, [setUpdateInfo, appSettings?.dismissed_update_version]);
+
+  const startPeriodicCheck = useCallback(() => {
+    if (intervalRef.current) return;
+    intervalRef.current = setInterval(() => {
+      if (appSettings?.auto_check_updates !== false) {
+        checkForUpdates();
+      }
+    }, CHECK_INTERVAL_MS);
+    return () => {
+      if (intervalRef.current) {
+        clearInterval(intervalRef.current);
+        intervalRef.current = null;
+      }
+    };
+  }, [checkForUpdates, appSettings?.auto_check_updates]);
+
+  return {
+    updateInfo,
+    appVersion,
+    loadVersion,
+    checkForUpdates,
+    startPeriodicCheck,
+  };
+}