Show elapsed-time cursor on usage bars

app/androidApp/src/test/kotlin/com/example/aiapp/SessionUsageTest.kt

@@ -1,5 +1,6 @@
 package com.example.aiapp
 
+import java.time.OffsetDateTime
 import kotlin.test.Test
 import kotlin.test.assertEquals
 
@@ -61,6 +62,52 @@ class SessionUsageTest {
         assertEquals(null, shortestUsageWindow(listOf(window("unknown", null))))
     }
 
+    @Test
+    fun `the time cursor follows elapsed time through the window`() {
+        val now = OffsetDateTime.parse("2026-09-17T12:00:00Z")
+
+        assertEquals(
+            0.4f,
+            usageWindowElapsedFraction(
+                window("5-hour window", 300, "2026-09-17T15:00:00Z"),
+                now,
+            ),
+        )
+    }
+
+    @Test
+    fun `the time cursor clamps at the window ends`() {
+        val now = OffsetDateTime.parse("2026-09-17T12:00:00Z")
+
+        assertEquals(
+            0f,
+            usageWindowElapsedFraction(
+                window("5-hour window", 300, "2026-09-17T18:00:00Z"),
+                now,
+            ),
+        )
+        assertEquals(
+            1f,
+            usageWindowElapsedFraction(
+                window("5-hour window", 300, "2026-09-17T11:00:00Z"),
+                now,
+            ),
+        )
+    }
+
+    @Test
+    fun `the time cursor is absent without a usable duration and reset time`() {
+        val now = OffsetDateTime.parse("2026-09-17T12:00:00Z")
+
+        assertEquals(null, usageWindowElapsedFraction(window("unknown", null), now))
+        assertEquals(null, usageWindowElapsedFraction(window("not running", 300), now))
+        assertEquals(
+            null,
+            usageWindowElapsedFraction(window("unreadable", 300, "not a timestamp"), now),
+        )
+        assertEquals(null, usageWindowElapsedFraction(window("zero", 0), now))
+    }
+
     private fun snapshot(
         machine: String,
         provider: String,
@@ -78,13 +125,13 @@ class SessionUsageTest {
             windows = emptyList(),
         )
 
-    private fun window(label: String, durationMinutes: Long?) =
+    private fun window(label: String, durationMinutes: Long?, resetsAt: String? = null) =
         UsageWindow(
             kind = "test",
             label = label,
             percent = 12.0,
             durationMinutes = durationMinutes,
-            resetsAt = null,
+            resetsAt = resetsAt,
             active = false,
         )
 }