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/test/java/me/khodak/claudeusage/UsageDataTest.kt

@@ -95,6 +95,32 @@ class UsageDataTest {
         assertFalse(UsageData().hasAnyReading)
     }
 
+    @Test
+    fun sessionReadingPctPrefersUtilization() {
+        // Utilization present → used verbatim (kept as Float, not truncated), even alongside messages.
+        val d = UsageData(fiveHourUtilization = 42.5f, messagesUsed = 10, messagesLimit = 100)
+        assertEquals(42.5f, d.sessionReadingPct, 0f)
+    }
+
+    @Test
+    fun sessionReadingPctFallsBackToMessages() {
+        // No utilization → derive from message counts so message-only accounts still get history.
+        val d = UsageData(messagesUsed = 30, messagesLimit = 120)
+        assertEquals(25f, d.sessionReadingPct, 0f)
+    }
+
+    @Test
+    fun sessionReadingPctMessagesCoercedTo100() {
+        val d = UsageData(messagesUsed = 150, messagesLimit = 100)
+        assertEquals(100f, d.sessionReadingPct, 0f)
+    }
+
+    @Test
+    fun sessionReadingPctNegativeWhenNoReading() {
+        // No utilization and no message data → -1f, so recordHistory skips the point.
+        assertEquals(-1f, UsageData().sessionReadingPct, 0f)
+    }
+
     @Test
     fun mergedWithEmptyFetchKeepsPreviousMetrics() {
         // (a) previous has a reading; this fetch is empty (all defaults) → previous metrics kept.