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/data/PreferencesManager.kt

@@ -74,6 +74,18 @@ class PreferencesManager(context: Context) {
 
     fun isLoggedIn(): Boolean = !getCookies().isNullOrBlank()
 
+    // Consecutive 401/403 counter — we only clear the session after several in a row, so a
+    // single transient auth failure (e.g. a Cloudflare challenge) doesn't log the user out.
+    fun getAuthFailCount(): Int = prefs.getInt(KEY_AUTH_FAILS, 0)
+    fun incAuthFailCount(): Int {
+        val n = getAuthFailCount() + 1
+        prefs.edit().putInt(KEY_AUTH_FAILS, n).apply()
+        return n
+    }
+    fun resetAuthFailCount() {
+        if (getAuthFailCount() != 0) prefs.edit().putInt(KEY_AUTH_FAILS, 0).apply()
+    }
+
     // ── Usage history (for the in-app chart) ─────────────────────────────────
 
     /**
@@ -140,6 +152,7 @@ class PreferencesManager(context: Context) {
         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
         private const val HISTORY_RETENTION_MS = 7 * 24 * 60 * 60 * 1000L  // keep 7 days