Add auto-updater (Phase 1 #4)

- tauri-plugin-updater + tauri-plugin-process registered (Rust + npm)
- Updater endpoint: github.com/hoornet/wrystr/releases/latest/download/latest.json
- Ed25519 signing keypair generated; public key in tauri.conf.json;
  private key added to TAURI_SIGNING_PRIVATE_KEY GitHub secret
- Release workflow passes TAURI_SIGNING_PRIVATE_KEY env var so
  tauri-action signs artifacts and generates latest.json manifest
- useUpdater hook: checks for updates 5 s after startup (silent on
  error); exposes available, version, install(), dismiss()
- UpdateBanner: dismissible top-of-app bar shown when an update is
  available — "Update & restart" downloads, installs, and relaunches

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -14,6 +14,8 @@
     "@tailwindcss/vite": "^4.2.1",
     "@tauri-apps/api": "^2",
     "@tauri-apps/plugin-opener": "^2",
+    "@tauri-apps/plugin-process": "^2.3.1",
+    "@tauri-apps/plugin-updater": "^2.10.0",
     "@types/dompurify": "^3.0.5",
     "dompurify": "^3.3.2",
     "marked": "^17.0.4",