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/build.gradle.kts

@@ -11,8 +11,8 @@ android {
         applicationId = "me.khodak.claudeusage"
         minSdk = 26
         targetSdk = 34
-        versionCode = 16
-        versionName = "1.15"
+        versionCode = 17
+        versionName = "1.16"
     }
 
     signingConfigs {