fix: ask for notifications in the extension too and use the extension notification API

[ttncode]

Summary

Fix notifications in the extension and make the setting clear.

Why 0.2.0 never asked in the extension: the manifest's notifications permission makes Chrome grant it up front, so the browser has nothing to ask and the first-run banner (which waited for a browser "default" state) never appeared.

• App-level on/off choice. Notifications now have their own setting on top of the browser permission. The first-run banner ("Get a notification when a focus session or break ends?" → Turn on) appears on first open in both the web app and the extension.
• Sample on turn-on. Turning them on sends a sample notification and shows a note: if it never arrived, allow notifications for the browser in system settings and turn off Do Not Disturb / Focus — the usual reason a granted notification is silently dropped.
• Extension uses chrome.notifications. Inside the extension, notifications go through the extension API (with the extension's icon); clicking one focuses the timer tab and window. The web app keeps the Notification API.
• Menu switch. "Notifications on · test" is replaced by a plain switch: Notifications: On / Off / Blocked. Blocked explains how to allow them in site settings.
• An earlier "Not now" dismissal is honoured as Off.

Testing

• Lint, typecheck, unit tests and build pass.
• Web (stubbed permission already granted): banner asks on first open; Turn on sends the sample and shows the note; the menu switch flips On/Off and stores the choice.
• Extension page (stubbed chrome API): banner asks; Turn on calls chrome.notifications.create with the sample and the extension icon; a 1-minute session ending sends "Focus session complete" the same way, with no web notification.