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/MainActivity.kt

@@ -1,9 +1,14 @@
 package me.khodak.claudeusage
 
+import android.Manifest
 import android.content.Intent
+import android.content.pm.PackageManager
+import android.os.Build
 import android.os.Bundle
 import android.view.View
+import androidx.activity.result.contract.ActivityResultContracts
 import androidx.appcompat.app.AppCompatActivity
+import androidx.core.content.ContextCompat
 import androidx.lifecycle.lifecycleScope
 import kotlinx.coroutines.launch
 import me.khodak.claudeusage.data.PreferencesManager
@@ -19,6 +24,9 @@ class MainActivity : AppCompatActivity() {
     private lateinit var prefs: PreferencesManager
     private lateinit var repo: UsageRepository
 
+    private val notifPermLauncher =
+        registerForActivityResult(ActivityResultContracts.RequestPermission()) { /* result handled silently */ }
+
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         binding = ActivityMainBinding.inflate(layoutInflater)
@@ -51,6 +59,8 @@ class MainActivity : AppCompatActivity() {
             })
         }
 
+        setupNotificationSettings()
+
         binding.btnDebug.setOnClickListener {
             if (binding.tvDebugInfo.visibility == android.view.View.GONE) {
                 binding.tvDebugInfo.text = repo.lastDebugInfo.ifBlank { "No debug info yet — tap Refresh first" }
@@ -75,6 +85,54 @@ class MainActivity : AppCompatActivity() {
         }
     }
 
+    private fun setupNotificationSettings() {
+        binding.switchNotify.isChecked = prefs.isNotifyEnabled()
+        binding.sliderSession.value = prefs.getSessionThreshold().toFloat().coerceIn(50f, 100f)
+        binding.sliderWeekly.value = prefs.getWeeklyThreshold().toFloat().coerceIn(50f, 100f)
+        applyThresholdLabels()
+        applyNotifyControlsEnabled(prefs.isNotifyEnabled())
+
+        binding.switchNotify.setOnCheckedChangeListener { _, checked ->
+            prefs.setNotifyEnabled(checked)
+            applyNotifyControlsEnabled(checked)
+            if (checked) requestNotificationPermission()
+        }
+        binding.sliderSession.addOnChangeListener { _, value, _ ->
+            prefs.setSessionThreshold(value.toInt())
+            applyThresholdLabels()
+        }
+        binding.sliderWeekly.addOnChangeListener { _, value, _ ->
+            prefs.setWeeklyThreshold(value.toInt())
+            applyThresholdLabels()
+        }
+
+        // Alerts default on, so prompt for the runtime permission once on first launch
+        // (a user who never toggles the switch would otherwise never be asked).
+        if (prefs.isNotifyEnabled()) requestNotificationPermission()
+    }
+
+    private fun applyThresholdLabels() {
+        binding.tvSessionThreshLabel.text = "Session alert at ${prefs.getSessionThreshold()}%"
+        binding.tvWeeklyThreshLabel.text = "Weekly alert at ${prefs.getWeeklyThreshold()}%"
+    }
+
+    private fun applyNotifyControlsEnabled(enabled: Boolean) {
+        binding.sliderSession.isEnabled = enabled
+        binding.sliderWeekly.isEnabled = enabled
+        val alpha = if (enabled) 1f else 0.4f
+        binding.tvSessionThreshLabel.alpha = alpha
+        binding.tvWeeklyThreshLabel.alpha = alpha
+    }
+
+    private fun requestNotificationPermission() {
+        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.TIRAMISU) return
+        if (ContextCompat.checkSelfPermission(this, Manifest.permission.POST_NOTIFICATIONS)
+            != PackageManager.PERMISSION_GRANTED
+        ) {
+            notifPermLauncher.launch(Manifest.permission.POST_NOTIFICATIONS)
+        }
+    }
+
     private fun refreshUsage() {
         binding.btnRefresh.isEnabled = false
         binding.progressIndicator.visibility = View.VISIBLE
@@ -87,6 +145,8 @@ class MainActivity : AppCompatActivity() {
                     ?: UsageData(errorMessage = "Network error")
             }
             prefs.saveUsageData(data)
+            prefs.recordHistory(data)
+            Notifier.checkAndNotify(this@MainActivity, prefs, data)
             updateUI(data)
             if (binding.tvDebugInfo.visibility == View.VISIBLE) {
                 binding.tvDebugInfo.text = repo.lastDebugInfo
@@ -157,6 +217,8 @@ class MainActivity : AppCompatActivity() {
 
         binding.tvError.text = data.errorMessage
         binding.tvError.visibility = if (data.errorMessage.isNotBlank()) View.VISIBLE else View.GONE
+
+        binding.historyChart.setData(prefs.getHistory())
     }
 
     private fun formatReset(epochMs: Long): String {