Fill the window on Linux; updates

- Linux: Tauri packs a window's webviews into one GTK box with equal
  shares and ignores their bounds, so the title bar took half the window.
  The title bar webview now has a fixed height and no share; the box's
  border is the resize frame (none when maximized).
- tauri-plugin-updater: checks llembas.eu/desktop/latest.json at start
  and every 6 h, an update button in the title bar and menu entries,
  .deb via pkexec, AppImage in place, then restart. Packages signed with
  the updater's minisign key; requireSignedVersion on.
  scripts/publish-update.sh signs a GitHub release and writes the manifest.

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

src-tauri/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "llembas-desktop"
-version = "0.2.0"
+version = "0.3.0"
 description = "LLeMbas Desktop: your LLeMbas server in a window, with the agent on this computer"
 authors = ["Jaroslav Beneš"]
 license = "MIT"
@@ -21,6 +21,7 @@ tauri = { version = "2", features = ["tray-icon", "unstable"] }
 tauri-plugin-dialog = "2"
 tauri-plugin-opener = "2"
 tauri-plugin-single-instance = "2"
+tauri-plugin-updater = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 ureq = { version = "2", default-features = false, features = ["tls", "native-certs", "json"] }
@@ -28,6 +29,10 @@ 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