v1.19: fix empty usage-history chart (always fetch utilization)

recordHistory() only ever stored fiveHourUtilization/weeklyUtilization, but
fetchUsage() returned early with message-count data — before calling the
/usage utilization endpoint — whenever the org JSON carried a message limit.
So utilization was never populated and the history chart stayed stuck on
'Collecting history…'. The prior chart fix only corrected the throttle.

- UsageRepository: always attempt /usage (preferred signal that drives the
  weekly bar + history); fall back to org message-count data only when
  utilization is unavailable.
- UsageData.sessionReadingPct: utilization preferred, message-count % fallback,
  -1f when no reading — so message-only accounts also build history.
- PreferencesManager.recordHistory: record the session line from
  sessionReadingPct instead of utilization-only.
- UsageDataTest: cover sessionReadingPct.

app/src/main/java/me/khodak/claudeusage/data/PreferencesManager.kt

@@ -94,13 +94,16 @@ class PreferencesManager(context: Context) {
     // ── Usage history (for the in-app chart) ─────────────────────────────────
 
     /**
-     * Append a history point if [data] carries a real utilization reading.
+     * Append a history point if [data] carries a real reading.
+     * The session line uses [UsageData.sessionReadingPct] (utilization preferred, message-count
+     * progress as fallback) so accounts that only expose message counts still build history.
      * De-duplicates rapid double-fires (manual refresh + background worker landing
      * together) by skipping points within [MIN_HISTORY_GAP_MS] of the last one, and
      * prunes anything older than [HISTORY_RETENTION_MS] / beyond [MAX_HISTORY_POINTS].
      */
     fun recordHistory(data: UsageData) {
-        if (data.fiveHourUtilization < 0f && data.weeklyUtilization < 0f) return
+        val sessionPct = data.sessionReadingPct
+        if (sessionPct < 0f && data.weeklyUtilization < 0f) return
         val now = System.currentTimeMillis()
         val history = getHistory().toMutableList()
         // Throttle to at most one point per MIN_HISTORY_GAP_MS by SKIPPING a too-soon reading.
@@ -111,7 +114,7 @@ class PreferencesManager(context: Context) {
         history.add(
             UsageSnapshot(
                 epochMs = now,
-                sessionPct = data.fiveHourUtilization,
+                sessionPct = sessionPct,
                 weeklyPct = data.weeklyUtilization
             )
         )