Add a floating focus Mini Timer

[caezium]

What

Adds an optional, always-on-top focus Mini Timer — a small floating pill that shows today's focused time and counts up live.

How it works

• Focused time is aggregated from analyzed timeline cards (everything that isn't Idle or Distraction), using the epoch start_ts/end_ts columns rather than the display text.
• Because analysis runs in ~15-min batches, the timer is live-optimistic: it takes the processed total and extends the most recent focused segment forward to now, but only while you're actively recording — not paused, and not idle (a non-latching CGEventSource system-idle check). When the next batch lands, the processed total absorbs the gap, so the number stays smooth. The forward extension is capped so a stalled pipeline can't run it away.

UX

• Non-activating NSPanel at floating level, joins all Spaces (incl. fullscreen), draggable, remembers its position. Off by default.
• Toggle from the menu bar (Show/Hide Focus Timer) or Settings → Other.

Files

• New: ProductivityStats, ProductivityPreferences, MiniTimerWindowController, MiniTimerView, and a small epoch-based fetchTimelineActivity query on StorageManager.
• Touched: AppDelegate (startup), StatusMenuView (menu toggle), SettingsOtherTabView (settings toggle).

Builds against main. The shared ProductivityStats/fetchTimelineActivity helper is intentionally small so a companion distraction-nudges change can reuse it.