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/tauri.conf.json

@@ -47,5 +47,14 @@
         ]
       }
     }
+  },
+  "plugins": {
+    "updater": {
+      "pubkey": "dW50cnVzdGVkIGNvbW1lbnQ6IG1pbmlzaWduIHB1YmxpYyBrZXkgMzNBNUJCMkNCQTBCOTEwRgpSV1FQa1F1NkxMdWxNOCtnT2NMQU5QOTBHZ3hUaC9UNHVFZThUR2lqOGdWWUVObi9mbU00amoyago=",
+      "endpoints": [
+        "https://llembas.eu/desktop/latest.json"
+      ],
+      "requireSignedVersion": true
+    }
   }
 }