v1.8: fix black screen on resume and crash on sync

- onResume() no longer triggers a refresh every time — only fetches when
  data is >5 min stale, so returning to app shows cached data instantly
  without a loading spinner
- Fix CancellationException being swallowed by catch(Exception) in
  refreshUsage(), which caused updates to run on a destroyed activity
- EncryptedSharedPreferences key invalidation (caused by enabling/changing
  biometrics or screen lock) now deletes the stale encrypted file and
  recreates it cleanly, rather than silently using empty fallback prefs
- Wrap all securePrefs read/write ops in try-catch so a mid-session
  Keystore failure degrades gracefully instead of crashing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/build.gradle.kts

@@ -11,8 +11,8 @@ android {
         applicationId = "me.khodak.claudeusage"
         minSdk = 26
         targetSdk = 34
-        versionCode = 8
-        versionName = "1.7"
+        versionCode = 9
+        versionName = "1.8"
     }
 
     signingConfigs {