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>

ui/titlebar.html

@@ -14,6 +14,7 @@
   <span class="bar__name" data-tauri-drag-region><b>LL</b>e<b>M</b>bas</span>
   <span class="bar__host" id="host" data-tauri-drag-region></span>
 
+  <button class="bar__update" id="update" type="button" hidden>update</button>
   <button class="bar__agent" id="agent" type="button" title="Agent">
     <span class="dot" aria-hidden="true"></span><span id="agent-text">agent off</span>
   </button>