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>

src-tauri/tauri.conf.json

@@ -33,5 +33,14 @@
       "icons/icon.icns",
       "icons/icon.ico"
     ]
+  },
+  "plugins": {
+    "updater": {
+      "endpoints": [
+        "https://github.com/hoornet/wrystr/releases/latest/download/latest.json"
+      ],
+      "pubkey": "dW50cnVzdGVkIGNvbW1lbnQ6IG1pbmlzaWduIHB1YmxpYyBrZXk6IDIwODYwNENEMjY2NEFCQkYKUldTL3EyUW16UVNHSUk2VXg5WXVZcDF2WUVFOU9kUWgyNjRsQzdoYjdwS0dra1VyUnhVK1EyY20K",
+      "dialog": false
+    }
   }
 }