Fix widget/app showing stale or no data; add live in-app refresh

Three reliability bugs made data inconsistent:

1. Empty-overwrite: a failed or partial fetch returned an empty
   UsageData that the worker/app saved unconditionally, wiping the last
   good reading and blanking the widget. Added UsageData.mergedWith()
   so a fetch that returns nothing usable keeps the previous snapshot,
   and a partial fetch falls back per-metric. Never blank again.

2. No in-app auto-refresh: onResume only refreshed when the cache was
   >5 min old and there was no live timer. Replaced with a foreground
   lifecycle loop that refreshes on open and every 30s while visible,
   always painting cached data first. Manual button keeps the spinner;
   the loop is silent. App refresh now also pushes the widget update.

3. Spurious logout: a single transient 401/403 (e.g. a Cloudflare
   challenge) called clearSession() immediately, logging the user out
   and showing "Not signed in". Now clears only after 3 consecutive
   auth failures; the counter resets on any successful read.

Battery-friendly: no foreground service. Background widget refresh
stays on the existing alarm + 15-min WorkManager, but with the merge
fix the widget always shows the last data it pulled.

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

app/src/main/java/me/khodak/claudeusage/UsageUpdateWorker.kt

@@ -31,8 +31,9 @@ class UsageUpdateWorker(
             val animJob = launch { rotateRefreshIcon() }
             try {
                 val data = UsageRepository(prefs).fetchUsage()
-                prefs.saveUsageData(data)
-                prefs.recordHistory(data)
+                // Preserve last-good data so a failed/partial fetch never blanks the widget.
+                prefs.saveUsageData(data.mergedWith(prefs.getUsageData()))
+                prefs.recordHistory(data)           // history records only fresh readings
                 Notifier.checkAndNotify(context, prefs, data)
             } catch (_: Exception) {}
             animJob.cancel()