feat: scaffold matrix_admin_panel v0.1.0 (issue 0163)

Wails + React + Mantine v7 admin panel for Matrix/Synapse. Replaces the
removed synapse-admin container. MAS OIDC PKCE login (loopback :8766) +
Synapse Admin API (users/rooms/sessions).

- MAS client: XSFD2SWA394DXRVJFTREAMY6J6 (public PKCE, no auth method).
- Backend: AdminService (Go) with Login/SetAdminToken/ListUsers/
  DeactivateUser/ResetUserPassword/ListRooms/DeleteRoom/GetUserDevices.
- Vendored helpers in internal/infra/ from registry:
  mas_oidc_loopback_go_infra, keyring_token_store_go_infra,
  synapse_admin_client_go_infra.
- Frontend: AppShell + sidebar tabs (Users/Rooms/Sessions). Sessions
  placeholder pending MAS admin API.
- Build verified: Linux + Windows.

.gitignore

@@ -0,0 +1,25 @@
+# Wails build artifacts
+build/bin/
+build/darwin/
+build/windows/
+build/linux/
+
+# Frontend
+frontend/dist/
+frontend/node_modules/
+frontend/wailsjs/go/
+frontend/wailsjs/runtime/
+
+# Local state
+*.db
+*.db-shm
+*.db-wal
+local_files/
+
+# IDE
+.vscode/
+.idea/
+
+# OS
+.DS_Store
+Thumbs.db