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>

app/src/main/AndroidManifest.xml

@@ -3,6 +3,7 @@
 
     <uses-permission android:name="android.permission.INTERNET" />
     <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
 
     <application
         android:allowBackup="false"