v1.16: simplify usage alerts to fixed 90% and 100% (less aggressive)

Replace the configurable threshold sliders with two fixed alert levels —
90% and 100% — per metric. Anti-spam now uses hysteresis instead of the
API reset-epoch (which could drift and re-fire): each level fires once
when crossed and re-arms only after usage drops back below it. Alerts are
posted only by the background worker, never the in-app refresh loop, so
you're not pinged while looking at the app. UI drops the sliders for a
one-line description; settings keep just the on/off switch.

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

app/src/main/java/me/khodak/claudeusage/data/PreferencesManager.kt

@@ -127,19 +127,13 @@ class PreferencesManager(context: Context) {
     fun isNotifyEnabled(): Boolean = prefs.getBoolean(KEY_NOTIFY_ENABLED, true)
     fun setNotifyEnabled(v: Boolean) = prefs.edit().putBoolean(KEY_NOTIFY_ENABLED, v).apply()
 
-    fun getSessionThreshold(): Int = prefs.getInt(KEY_NOTIFY_SESSION_PCT, 90)
-    fun setSessionThreshold(pct: Int) = prefs.edit().putInt(KEY_NOTIFY_SESSION_PCT, pct).apply()
-
-    fun getWeeklyThreshold(): Int = prefs.getInt(KEY_NOTIFY_WEEKLY_PCT, 85)
-    fun setWeeklyThreshold(pct: Int) = prefs.edit().putInt(KEY_NOTIFY_WEEKLY_PCT, pct).apply()
-
     /**
-     * Tracks the reset-epoch a metric was last notified for, so we alert at most once
-     * per limit window. When the window rolls over (reset epoch changes), it re-arms.
+     * Per-level "already alerted" flag (e.g. "session_90"). Set when the level is crossed,
+     * cleared when usage drops back below it — so each level fires once per window.
      */
-    fun getNotifiedResetEpoch(key: String): Long = prefs.getLong("notified_$key", 0L)
-    fun setNotifiedResetEpoch(key: String, epoch: Long) =
-        prefs.edit().putLong("notified_$key", epoch).apply()
+    fun wasNotified(key: String): Boolean = prefs.getBoolean("notified_$key", false)
+    fun setNotified(key: String, v: Boolean) =
+        prefs.edit().putBoolean("notified_$key", v).apply()
 
     companion object {
         private const val KEY_COOKIES = "session_cookies"
@@ -150,8 +144,6 @@ class PreferencesManager(context: Context) {
         private const val KEY_ACTIVE_MASK = "active_mask"
         private const val KEY_HISTORY = "usage_history"
         private const val KEY_NOTIFY_ENABLED = "notify_enabled"
-        private const val KEY_NOTIFY_SESSION_PCT = "notify_session_pct"
-        private const val KEY_NOTIFY_WEEKLY_PCT = "notify_weekly_pct"
         private const val KEY_AUTH_FAILS = "auth_fail_count"
 
         private const val MIN_HISTORY_GAP_MS = 2 * 60 * 1000L      // collapse readings <2 min apart