Merging fix for Android file I/O permissions

[kevincarlson]

Android's Storage Access Framework (SAF) grants only a temporary permission for a content:// URI when the user picks a file via the file picker. That permission is tied to the app process and is revoked when the app is killed. Attempting to re-open the same URI from Recents in a new session therefore fails with a permissions error.

Fix — 4 changes:

1. UriPermissionPlugin.kt (new) — A Tauri Android plugin that calls ContentResolver.takePersistableUriPermission(uri, READ | WRITE). Android will then persist that grant across app restarts in its own permission store.
2. MainActivity.kt — Registers the plugin via registerPlugin(UriPermissionPlugin::class.java) so Tauri routes the JS invoke call to it.
3. commands.ts — Adds takePersistableUriPermission(uri), a typed TypeScript wrapper around invoke('plugin:uriPermission|takePersistablePermission', { uri }).
4. useFileOperations.ts — Calls takePersistableUriPermission in two places:
   • loadDocument() — when a content:// file is opened (covers the file picker path and the Recents path on first open)
   • handleSaveAs() — when a new file is saved to a content:// destination

Both calls are wrapped in try/catch so they are silently ignored on desktop and on URIs that don't support persistable permissions.