Widget: add postpone (tomorrow/next week/next month) to task detail popup

Alongside Complete/Edit, doot-native tasks now get a Postpone button with
a dropdown (Tomorrow / Next week / Next month), reusing the existing
reschedule wiring (WidgetRepository.reschedule) the due-date picker
already uses.

Caught and documented a real divergence while writing the test: Java's
LocalDate.plusMonths CLAMPS to the target month's last valid day (Jan 31
-> Feb 28), while the Go server's ComputeNextOccurrence (recurrence
math) OVERFLOWS instead (Jan 31 + 1 month -> Mar 3) via time.AddDate.
Verified by actually running both, not assumed -- a first draft of this
test asserted the wrong (Go-style) behavior before checking. Not
reconciled here, just accurately documented as a known inconsistency
between this client-side helper and the server's date math.

4 new tests, all passing.

2 files changed, 76 insertions, 0 deletions

diff --git a/android/app/src/main/java/org/terst/doot/widget/ui/TaskDetailActivity.kt b/android/app/src/main/java/org/terst/doot/widget/ui/TaskDetailActivity.kt
index f486dec..effb535 100644
--- a/android/app/src/main/java/org/terst/doot/widget/ui/TaskDetailActivity.kt
+++ b/android/app/src/main/java/org/terst/doot/widget/ui/TaskDetailActivity.kt
@@ -441,6 +441,31 @@ fun TaskDetailSheet(
                             border = androidx.compose.foundation.BorderStroke(1.dp, Color.White.copy(alpha = 0.3f))
                         ) { Text("Edit") }
                     }
+                    if (isDoot && completable) {
+                        Box(modifier = Modifier.weight(1f)) {
+                            var showPostponeMenu by remember { mutableStateOf(false) }
+                            OutlinedButton(
+                                onClick = { showPostponeMenu = true },
+                                modifier = Modifier.fillMaxWidth(),
+                                colors = ButtonDefaults.outlinedButtonColors(contentColor = Color.White),
+                                border = androidx.compose.foundation.BorderStroke(1.dp, Color.White.copy(alpha = 0.3f))
+                            ) { Text("Postpone") }
+                            DropdownMenu(expanded = showPostponeMenu, onDismissRequest = { showPostponeMenu = false }) {
+                                DropdownMenuItem(
+                                    text = { Text("Tomorrow") },
+                                    onClick = { showPostponeMenu = false; onReschedule(postponeDate(PostponePeriod.TOMORROW)) }
+                                )
+                                DropdownMenuItem(
+                                    text = { Text("Next week") },
+                                    onClick = { showPostponeMenu = false; onReschedule(postponeDate(PostponePeriod.NEXT_WEEK)) }
+                                )
+                                DropdownMenuItem(
+                                    text = { Text("Next month") },
+                                    onClick = { showPostponeMenu = false; onReschedule(postponeDate(PostponePeriod.NEXT_MONTH)) }
+                                )
+                            }
+                        }
+                    }
                     if (completable) {
                         Button(
                             onClick = onComplete,
@@ -493,6 +518,18 @@ internal fun isoDateFromMillis(millis: Long): String {
     return "%04d-%02d-%02d".format(cal.get(Calendar.YEAR), cal.get(Calendar.MONTH) + 1, cal.get(Calendar.DAY_OF_MONTH))
 }
 
+internal enum class PostponePeriod { TOMORROW, NEXT_WEEK, NEXT_MONTH }
+
+/** ISO date (yyyy-MM-dd) for the postpone menu, relative to the device's current date. */
+internal fun postponeDate(period: PostponePeriod, today: LocalDate = LocalDate.now()): String {
+    val target = when (period) {
+        PostponePeriod.TOMORROW -> today.plusDays(1)
+        PostponePeriod.NEXT_WEEK -> today.plusWeeks(1)
+        PostponePeriod.NEXT_MONTH -> today.plusMonths(1)
+    }
+    return target.toString()
+}
+
 private val LABEL_COLOR_PALETTE = listOf(
     "#3B82F6", "#EF4444", "#F59E0B", "#10B981", "#8B5CF6", "#EC4899", "#6B7280"
 )
diff --git a/android/app/src/test/java/org/terst/doot/widget/ui/TaskDetailActivityTest.kt b/android/app/src/test/java/org/terst/doot/widget/ui/TaskDetailActivityTest.kt
new file mode 100644
index 0000000..94a5ce9
--- /dev/null
+++ b/android/app/src/test/java/org/terst/doot/widget/ui/TaskDetailActivityTest.kt
@@ -0,0 +1,39 @@
+package org.terst.doot.widget.ui
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import java.time.LocalDate
+
+class TaskDetailActivityTest {
+
+    private val anchor = LocalDate.of(2026, 8, 6) // Thursday
+
+    @Test
+    fun `postponeDate tomorrow is a single day ahead`() {
+        assertEquals("2026-08-07", postponeDate(PostponePeriod.TOMORROW, anchor))
+    }
+
+    @Test
+    fun `postponeDate next week is seven days ahead`() {
+        assertEquals("2026-08-13", postponeDate(PostponePeriod.NEXT_WEEK, anchor))
+    }
+
+    @Test
+    fun `postponeDate next month is a calendar month ahead`() {
+        assertEquals("2026-09-06", postponeDate(PostponePeriod.NEXT_MONTH, anchor))
+    }
+
+    @Test
+    fun `postponeDate next month across a short month clamps -- NOT the same as the Go server's overflow behavior`() {
+        // Verified, not assumed: java.time.LocalDate.plusMonths CLAMPS to the target
+        // month's last valid day (Jan 31 -> Feb 28), unlike
+        // ComputeNextOccurrence (internal/models/recurrence.go)'s Go time.AddDate,
+        // which OVERFLOWS instead (Jan 31 + 1 month -> Mar 3). This is a real,
+        // known divergence between this client-side postpone helper and the
+        // server's recurrence date math for day-of-month 29-31 -- not something
+        // this fix reconciles, just documenting it accurately instead of
+        // asserting the wrong behavior.
+        val jan31 = LocalDate.of(2026, 1, 31)
+        assertEquals("2026-02-28", postponeDate(PostponePeriod.NEXT_MONTH, jan31))
+    }
+}