feat: login MAS OIDC end-to-end (issue 0147)

Backend Go:
- MatrixService with Login/GetSession/Logout bindings
- Uses 3 registry helpers via go.work:
  - mas_oidc_loopback_go_infra (PKCE flow)
  - keyring_token_store_go_infra (SO keychain)
  - matrix_client_init_go_infra (mautrix client)
- whoami helper to discover user_id+device_id pre-init

Frontend React+Vite+TS:
- Mantine v7 + @tabler/icons-react
- LoginScreen with 'Sign in with Matrix' button
- HomeScreen with profile card + logout
- Dark theme violet accent
- @mantine/notifications wired

Wails config:
- Switched to pnpm (workspace: protocol)
- Bindings auto-generated for MatrixService
- 1280x800 default window

Build: 68MB linux/amd64 binary in build/bin/

wails.json

@@ -2,12 +2,12 @@
   "$schema": "https://wails.io/schemas/config.v2.json",
   "name": "matrix_client_pc",
   "outputfilename": "matrix_client_pc",
-  "frontend:install": "npm install",
-  "frontend:build": "npm run build",
-  "frontend:dev:watcher": "npm run dev",
+  "frontend:install": "pnpm install",
+  "frontend:build": "pnpm build",
+  "frontend:dev:watcher": "pnpm dev",
   "frontend:dev:serverUrl": "auto",
   "author": {
     "name": "Egutierrez",
     "email": "egutierrez@dead.dd"
   }
-}
+}