Configurable settings: scan interval, model, roast language

[Amayyas]

Add a settings screen exposing:

• Scan interval (currently hardcoded 5 min via chrome.alarms).
• Model selection (ties into the multi-provider work, Support any API provider (multi-provider keys) #2).
• Roast language (default: follow the browser locale).

Persist everything in chrome.storage.local and read it in the service worker / aiClient.