Add non-intrusive auto-updater

Tauri updater + process plugins. Checks Gitea Releases endpoint a few
seconds after launch; on a newer signed version shows a dismissible
bottom-right banner with download progress and auto-relaunch. Public
signing key in tauri.conf.json; release flow documented in RELEASING.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -13,6 +13,8 @@
     "@tauri-apps/api": "^2",
     "@tauri-apps/plugin-dialog": "^2.7.1",
     "@tauri-apps/plugin-opener": "^2",
+    "@tauri-apps/plugin-process": "^2.3.1",
+    "@tauri-apps/plugin-updater": "^2.10.1",
     "react": "^19.1.0",
     "react-dom": "^19.1.0",
     "three": "^0.185.1"