feat(widget): open the app from the widget headers (#18, #20)

Tapping a widget header now opens the app on a sensible top-level view,
via a shared WidgetNavRequest.OpenView(view?) — a concrete view roots
there over the default home, a null view resolves to the default home:

- Month widget: the month/year title opens the month view (#18). Its
  "today" button now snaps the grid back to the current month in place
  (the reset that used to sit on the title), so paging + jump-to-today
  are both reachable and the title is free to open the app.
- Agenda widget: the "Upcoming" title opens the default view (#20), so
  users whose home view is Week/Month can reach it in one tap instead of
  drilling through a day or event.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/java/de/jeanlucmakiola/calendula/MainActivity.kt

@@ -193,6 +193,12 @@ class MainActivity : AppCompatActivity() {
         // An external date tap (launcher/clock) has no widget source, so it opens
         // the day view rooted over the default home view (OpenDate source = null).
         calendarTimeDateOrNull()?.let { return WidgetNavRequest.OpenDate(it.toString(), source = null) }
+        // A widget header tap: open a top-level view with no date drill-in. The
+        // empty string carried by [openViewIntent] means "the default home view".
+        if (hasExtra(EXTRA_OPEN_VIEW)) {
+            val name = getStringExtra(EXTRA_OPEN_VIEW).orEmpty()
+            return WidgetNavRequest.OpenView(CalendarView.entries.firstOrNull { it.name == name })
+        }
         val source = sourceViewOrNull()
         val eventId = getLongExtra(EXTRA_EVENT_ID, -1L)
         return when {
@@ -244,6 +250,10 @@ class MainActivity : AppCompatActivity() {
         private const val EXTRA_DATE_ISO = "de.jeanlucmakiola.calendula.extra.DATE_ISO"
         private const val EXTRA_CREATE = "de.jeanlucmakiola.calendula.extra.CREATE"
 
+        // A widget header tap asking to open a top-level view (no date drill-in).
+        // Its value is the target [CalendarView] name, or "" for the default view.
+        private const val EXTRA_OPEN_VIEW = "de.jeanlucmakiola.calendula.extra.OPEN_VIEW"
+
         // The [CalendarView] (by name) of the widget a launch came from. Roots the
         // in-app back stack in that view; absent for non-widget launches (reminders).
         private const val EXTRA_SOURCE_VIEW = "de.jeanlucmakiola.calendula.extra.SOURCE_VIEW"
@@ -307,5 +317,19 @@ class MainActivity : AppCompatActivity() {
                 putExtra(EXTRA_DATE_ISO, date.toString())
                 addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
             }
+
+        /**
+         * Open the app on a top-level [view] with no date drill-in — a widget
+         * header tap. A null [view] opens the user's default home view (the agenda
+         * widget's "Upcoming" title); a concrete view roots there over the default
+         * home (the month widget's month/year title → [CalendarView.Month]). The
+         * per-view data URI keeps distinct headers' PendingIntents from collapsing.
+         */
+        fun openViewIntent(context: Context, view: CalendarView?): Intent =
+            Intent(context, MainActivity::class.java).apply {
+                data = "calendula://view/${view?.name ?: "default"}".toUri()
+                putExtra(EXTRA_OPEN_VIEW, view?.name ?: "")
+                addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)
+            }
     }
 }

app/src/main/java/de/jeanlucmakiola/calendula/ui/CalendarHost.kt

@@ -212,6 +212,17 @@ fun CalendarHost(
                 detailKey = key
                 onWidgetNavConsumed()
             }
+            is WidgetNavRequest.OpenView -> {
+                // A widget header tap: land on a top-level view with no date
+                // drill-in. Reveal it by dropping any covering overlay, then root
+                // the stack on the target (null → the default home) over the
+                // default home — so backing out returns to the default, then exits.
+                dismissCoveringOverlays()
+                createDateIso = null
+                pendingDayIso = null
+                viewStack = viewBaseStack(defaultView, req.view ?: defaultView)
+                onWidgetNavConsumed()
+            }
             is WidgetNavRequest.Create -> {
                 // External "new event" entries (QS tile / launcher shortcut /
                 // widget) must land on top of whatever is open — the form overlay

app/src/main/java/de/jeanlucmakiola/calendula/ui/WidgetNavRequest.kt

@@ -31,4 +31,13 @@ sealed interface WidgetNavRequest {
 
     /** Open the create-event form prefilled for [dateIso] (today when null). */
     data class Create(val dateIso: String?) : WidgetNavRequest
+
+    /**
+     * Open the app rooted on a top-level [view] with no date drill-in — a widget
+     * header tap. A null [view] means "the user's default home view" (the agenda
+     * widget's "Upcoming" title, issue #20); a concrete view roots there over the
+     * default home (the month widget's month/year title → [CalendarView.Month],
+     * issue #18), so backing out returns to the default view, then exits.
+     */
+    data class OpenView(val view: CalendarView?) : WidgetNavRequest
 }

app/src/main/java/de/jeanlucmakiola/calendula/widget/agenda/AgendaWidget.kt

@@ -193,6 +193,8 @@ private fun AgendaHeader() {
         modifier = GlanceModifier.fillMaxWidth().padding(horizontal = 4.dp),
         verticalAlignment = Alignment.CenterVertically,
     ) {
+        // Tap the "Upcoming" title to open the app on the user's default view
+        // (issue #20) — a null target resolves to the default home in the host.
         Text(
             text = context.getString(R.string.widget_agenda_title),
             style = TextStyle(
@@ -200,7 +202,11 @@ private fun AgendaHeader() {
                 fontSize = 16.sp,
                 fontWeight = FontWeight.Medium,
             ),
-            modifier = GlanceModifier.defaultWeight(),
+            modifier = GlanceModifier
+                .defaultWeight()
+                .clickable(
+                    actionStartActivity(MainActivity.openViewIntent(context, view = null)),
+                ),
         )
         IconButton(
             resId = R.drawable.ic_widget_refresh,

app/src/main/java/de/jeanlucmakiola/calendula/widget/month/MonthWidget.kt

@@ -194,6 +194,8 @@ private fun MonthHeader(label: String) {
                 ),
             ),
         )
+        // Tap the month/year title to open the app on the month view (issue #18),
+        // rooted over the default home so backing out returns there.
         Text(
             text = label,
             style = TextStyle(
@@ -204,16 +206,17 @@ private fun MonthHeader(label: String) {
             ),
             modifier = GlanceModifier
                 .defaultWeight()
-                .clickable(actionRunCallback<ResetMonthAction>()),
+                .clickable(
+                    actionStartActivity(
+                        MainActivity.openViewIntent(context, CalendarView.Month),
+                    ),
+                ),
         )
+        // The "today" button snaps the grid back to the current month in place.
         HeaderIcon(
             resId = R.drawable.ic_widget_today,
             contentDescription = context.getString(R.string.widget_today),
-            onClick = GlanceModifier.clickable(
-                actionStartActivity(
-                    MainActivity.openDateIntent(context, today(systemZone()), CalendarView.Month),
-                ),
-            ),
+            onClick = GlanceModifier.clickable(actionRunCallback<ResetMonthAction>()),
         )
         HeaderIcon(
             resId = R.drawable.ic_widget_chevron_right,