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/java/me/khodak/claudeusage/data/UsageSnapshot.kt

@@ -0,0 +1,11 @@
+package me.khodak.claudeusage.data
+
+/**
+ * A single point-in-time reading of usage, stored for the in-app history chart.
+ * Percentages are 0-100; -1 means "no reading for this metric at this time".
+ */
+data class UsageSnapshot(
+    val epochMs: Long = 0L,
+    val sessionPct: Float = -1f,
+    val weeklyPct: Float = -1f
+)