feat(core,gui): tray states and menu

The tray icon follows the drive's phase (synced, syncing, paused,
offline, needs attention) and reports NeedsAttention while changes keep
failing. The menu gains an issues item that opens the Sync page, Sign In
when signed out, Hide Tray Icon (persisted as tray_hidden) and Stop
Proton Drive, which pdfs-app confirms via --confirm-stop. A second
pdfs-app launch raises the running window.

docs/CHANGELOG.md

@@ -35,6 +35,14 @@ Schema: **31** (unchanged).
   shared by every transfer and applied to running ones from their next block; downloads include
   reads through the mount. `pdfs sync limit --up 2M --down 0` does the same, and `pdfs sync limit`
   on its own prints the current caps. The caps are kept in `config.json` across restarts.
+- **Tray states and menu.** The tray icon shows whether the drive is synced, syncing, paused,
+  offline or needs attention. It asks the panel for attention while changes keep failing, and
+  its tooltip gives the same line as the menu. A left click opens the app. The menu gains
+  "N issues — View", which opens the Sync page, and "Sign In…" when signed out. "Hide Tray Icon"
+  is remembered across logins (`tray_hidden` in `config.json`), and Preferences → Appearance
+  → Show tray icon brings the icon back. "Stop Proton Drive…" asks for confirmation in the app,
+  then stops the service and the tray. `pdfs-app` accepts `--page <name>` and `--confirm-stop`.
+  A second launch raises the running window instead of opening another one.
 
 ### Changed
 - **New app shell.** Every page has its own header bar; the sidebar ends in a footer with sync