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>

CHANGELOG.md

@@ -6,6 +6,18 @@ All notable changes to LLeMbas Desktop. The format follows
 
 ## [Unreleased]
 
+### Added
+
+- Updates: the app looks for a new release at start and every six hours, shows an **update**
+  button in the title bar when there is one, and installs it (the .deb with the system's password
+  prompt, an AppImage in place) after checking its signature. The menu can also check by hand.
+
+### Fixed
+
+- On Linux the server's page took only the lower half of the window, under an empty title bar
+  area. The title bar is now its fixed height and the page takes the rest; the frame you resize
+  the window by is drawn around both.
+
 ## [0.2.0] — 2026-10-14
 
 ### Added