feat: scaffold matrix_client_pc (Wails + React-TS + mautrix-go + keyring)

Initial scaffold for issue 0147. App lives at
projects/element_agents/apps/matrix_client_pc/.

Deps:
- github.com/wailsapp/wails/v2 v2.11.0
- maunium.net/go/mautrix v0.25.2
- github.com/zalando/go-keyring v0.2.6

Next: MatrixService bindings + Login/Home React screens + helpers
(matrix_client_init, mas_oidc_loopback, keyring_token_store).

.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