feat(extension): add browser extension native messaging foundation

[apzuk3]

Summary

• scaffold a single-source browser extension package with WXT for Chromium and Firefox, including a background bridge that uses native messaging and websocket reconnect logic
• add native messaging host mode to the desktop app and implement stdio message handling that fetches extension connection info from the local focusd web server
• wire /extension/bootstrap and authenticated /extension/ws routes on the fixed local server (:50533) with per-session API key generation and constant-time validation
• update native messaging manifest installation to generate a launcher script that starts the app in native host mode for both Chrome and Firefox manifests

Validation

• go test ./internal/extension ./internal/nativemessaging
• go test ./... (root package fails in this environment because frontend/dist is not present for go:embed)