feat: phase 5 distribution — tauri shell, pwa, desktop release artifacts

- apps/desktop: Tauri 2 thin shell; connect screen stores the deployment
  URL, webview renders the remote console, tray with Show/Change Server/
  Quit, close-to-tray, single-instance
- frontend: vite-plugin-pwa generates manifest + Workbox SW (shell
  precache, /api /auth /mailpit NetworkOnly), install banner on
  beforeinstallprompt, rasterized PWA icons
- release.yml: desktop bundle matrix (linux/macos/windows) attached to
  GitHub Releases
- workspace 0.5.0; Dockerfiles copy the desktop workspace manifest

Authored By: TDvorak <[email protected]>

apps/frontend/public/icon-maskable.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64">
+  <rect width="64" height="64" fill="#f4693a"/>
+  <g fill="#ffffff" transform="translate(6.4 6.4) scale(0.8)">
+    <rect x="15" y="14" width="34" height="10" rx="5"/>
+    <rect x="15" y="27" width="23" height="10" rx="5" opacity="0.7"/>
+    <rect x="15" y="40" width="10" height="10" rx="5"/>
+  </g>
+</svg>