v1.14: usage history chart + threshold notifications

Add an in-app 7-day history chart and opt-in usage alerts, the two
features requested from the macOS Claude-Usage-Tracker that map cleanly
to an Android widget app.

History:
- UsageSnapshot model; PreferencesManager records session/weekly
  utilization on every refresh (7-day retention, <=600 points, collapses
  readings under 2 min apart to avoid worker+manual double-logging).
- HistoryChartView: dependency-free Canvas line chart (session/weekly,
  0/50/100% gridlines), breaks the line across >35-min gaps.
- New HISTORY card with chart + legend.

Notifications:
- Notifier posts when session/weekly crosses a user threshold, at most
  once per limit window (keyed on reset-epoch, re-arms on rollover).
- USAGE ALERTS card: enable switch + session/weekly sliders (50-100%,
  defaults 90/85). POST_NOTIFICATIONS permission + runtime request.
- Wired into the existing 5-min background worker.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -15,6 +15,10 @@ Android home screen widget that shows your Claude Pro usage at a glance.
   yellow → orange → red → purple (burning way too fast), with an "X% over/under pace" label.
 - **Peak-hours indicator** — a Claude burst icon that lights up 🔥 during Anthropic's peak window
   (5–11 AM Pacific, Mon–Fri), when tokens burn faster, with a countdown to the window close.
+- **Usage history chart** — the app plots your session and weekly utilization over the past 7 days,
+  so you can see your consumption trend, not just the current snapshot.
+- **Usage alerts** — opt-in notifications when session or weekly usage crosses a threshold you set
+  (sliders, 50–100%). Each alert fires at most once per limit window, so you're never spammed.
 - Tap the widget to open the app; tap ⟳ to force-refresh
 - Responsive: works as 4×1 (compact) or 4×2 (full)
 - Auto-refreshes every 5 minutes in the background