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/build.rs

@@ -1,5 +1,5 @@
-//! Besides Tauri's own build step: the two themes' colours the window itself paints, read out
-//! of `ui/tokens.css` (the server's tokens, fetched by `scripts/fetch-tokens.sh`) into
+//! Besides Tauri's own build step: the two themes' page colour, which the window itself paints
+//! before the page has (so a dark theme does not start white), read out of `ui/tokens.css` (the server's tokens, fetched by `scripts/fetch-tokens.sh`) into
 //! `$OUT_DIR/tokens.rs`, so no colour is written in Rust. `lib.rs` includes that file.
 
 use std::env;
@@ -10,7 +10,7 @@ use std::path::Path;
 /// Rust names the constants get.
 const THEMES: [(&str, &str); 2] = [("moria", "MORIA"), ("shire", "SHIRE")];
 /// The tokens taken from each theme, by the custom property and the constant's suffix.
-const TOKENS: [(&str, &str); 2] = [("--bg", "BG"), ("--border", "BORDER")];
+const TOKENS: [(&str, &str); 1] = [("--bg", "BG")];
 
 fn main() {
     let tokens = Path::new("../ui/tokens.css");