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/src/main/java/me/khodak/claudeusage/MainActivity.kt

@@ -68,7 +68,10 @@ class MainActivity : AppCompatActivity() {
         val cached = prefs.getUsageData()
         updateUI(cached)
         if (prefs.isLoggedIn()) {
-            refreshUsage()
+            val staleMs = 5 * 60 * 1000L
+            if ((cached?.lastUpdated ?: 0L) < System.currentTimeMillis() - staleMs) {
+                refreshUsage()
+            }
         }
     }
 
@@ -79,6 +82,7 @@ class MainActivity : AppCompatActivity() {
             val data = try {
                 repo.fetchUsage()
             } catch (e: Exception) {
+                if (e is kotlinx.coroutines.CancellationException) throw e
                 prefs.getUsageData()?.copy(errorMessage = "Network error")
                     ?: UsageData(errorMessage = "Network error")
             }