One window, the app's own page; the server through /api/v2

The two webviews (local title bar, remote web UI) become one WebviewWindow on
ui/index.html. The window refuses every navigation but the app's page and its
sealed frames; the capability still names only main and no remote URL.

- server.rs: discovery refuses a server older than 2.0 (protocol 3 and major
  version 2) and an app API newer than this app knows; the app's own device
  login for the app scope, the token kept in account.json (0600) and never
  given to the page; a proxy limited to /api/v2 paths; the SSE stream.
- files.rs: list and read the agent's working directory, resolved (links and
  .. included) and refused outside the agent's folders.
- sealed.rs: HTML for the viewer on its own scheme under the server's sealed
  policy: no bridge, no network, no navigation.
- watch.rs + tauri-plugin-notification: a notification when a reply or an
  agent chat finishes out of sight.
- A failed start and panics go to desktop.log; a missing tray no longer stops
  the app; the DMA-BUF renderer is off under NVIDIA's driver.
- The unstable multiwebview feature, the gtk dependency and the theme report
  are gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src-tauri/capabilities/default.json

@@ -1,8 +1,10 @@
 {
   "$schema": "../gen/schemas/desktop-schema.json",
   "identifier": "default",
-  "description": "The app's own pages: the title bar and the first-run page. The server's web UI is a remote page in the same window and gets no IPC at all: this capability lists no remote URL. The window permissions are the title bar's: moving, resizing, minimising, maximising and closing (which hides).",
-  "windows": ["main"],
+  "description": "The app's own page (ui/), the only content with the bridge. It names the main window and no remote URL: no server page is ever loaded, and HTML the viewer shows is a sealed frame on its own scheme, which no capability names. The window permissions are the title bar's: moving, resizing, minimising, maximising and closing (which hides).",
+  "windows": [
+    "main"
+  ],
   "permissions": [
     "core:default",
     "core:window:allow-start-dragging",