feat: client auto-update with Ed25519 signing and dynamic server URL

- Add tauri-plugin-updater and tauri-plugin-process for in-app updates
- Rust commands (check_client_update, download_and_install_update) build
  updater with dynamic endpoint at runtime for self-hosted compatibility
- Server endpoint GET /api/v1/client-update/{target}/{version} translates
  GitHub Releases into Tauri updater JSON format with .sig content
- UpdateNotifier banner component with install/dismiss controls
- CI workflow produces signed .nsis.zip + .sig updater artifacts
- Self-signed TLS support via dangerousAcceptInvalidCerts config

Client/tauri-client/src-tauri/tauri.conf.json

@@ -42,5 +42,15 @@
       }
     }
   },
-  "plugins": {}
+  "plugins": {
+    "updater": {
+      "pubkey": "dW50cnVzdGVkIGNvbW1lbnQ6IG1pbmlzaWduIHB1YmxpYyBrZXk6IDgxMkZGMTUzMDBBNkFCNDAKUldSQXE2WUFVL0V2Z2NjekFXaVI1elpQYVBmOUdKNmZrTzZwRC80RHlBMkQyYzZWYXdTK00xK0wK",
+      "endpoints": [],
+      "dangerousAcceptInvalidCerts": true,
+      "dangerousAcceptInvalidHostnames": true,
+      "windows": {
+        "installMode": "passive"
+      }
+    }
+  }
 }