Add local website time tracking and focus sessions

Summary

Add opt-in local website time reports and focus sessions.

Scope

• Count time for the active page while Photon is the foreground app, grouped by site and local time period.
• Build time segments from visibility/focus/tab transitions rather than a high-frequency polling timer.
• Provide a local report and focus timer for sites the user chooses to block during a session.
• Exclude private browsing and include saved time data in browsing-data clearing.
• Keep all records on-device and avoid collecting full URLs when site-level totals are sufficient.

Acceptance criteria

• Time accounting handles tab switches, window focus changes, sleep, and midnight boundaries.
• Focus restrictions are checked before navigation through an explicit Photon policy path.
• Private sessions leave no saved time or focus records.
• The feature is opt-in and can be cleared from Privacy settings.

Design reference: Zephium’s time model uses transition-based accounting, local aggregates, and explicit focus gates: https://github.com/zephium-browser/Zephium/blob/main/crates/zephium-core/src/time.rs