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/Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "llembas-desktop"
 version = "0.7.2"
-description = "LLeMbas Desktop: your LLeMbas server in a window, with the agent on this computer"
+description = "LLeMbas Desktop: the agent on this computer and your LLeMbas server's chats, in an app of its own"
 authors = ["Jaroslav Beneš"]
 license = "MIT"
 repository = "https://github.com/LLeMbas/LLeMbas-Desktop"
@@ -17,8 +17,9 @@ crate-type = ["staticlib", "cdylib", "rlib"]
 tauri-build = { version = "2", features = [] }
 
 [dependencies]
-tauri = { version = "2.12", features = ["tray-icon", "unstable"] }
+tauri = { version = "2.12", features = ["tray-icon"] }
 tauri-plugin-dialog = "2"
+tauri-plugin-notification = "2"
 tauri-plugin-opener = "2"
 tauri-plugin-single-instance = "2"
 tauri-plugin-updater = "2"
@@ -29,10 +30,6 @@ ureq = { version = "2", default-features = false, features = ["tls", "native-cer
 [target.'cfg(unix)'.dependencies]
 libc = "0.2"
 
-[target.'cfg(target_os = "linux")'.dependencies]
-# The window's GTK box: Tauri packs every webview into it with an equal share on Linux.
-gtk = "0.18"
-
 [profile.release]
 codegen-units = 1
 lto = true