feat(intent): create events from external ACTION_INSERT launches (#30)

Register an intent-filter for ACTION_INSERT on the events dir mime type
(vnd.android.cursor.dir/event), the way the AOSP calendar accepts one, so
other apps and widgets (e.g. the Todo Agenda widget) can launch Calendula
to create a new event.

MainActivity.insertFormOrNull parses the standard CalendarContract extras
(EXTRA_EVENT_BEGIN_TIME/END_TIME/ALL_DAY, Events.TITLE/DESCRIPTION/
EVENT_LOCATION/RRULE) into a prefilled EventForm via the pure, unit-tested
buildInsertEventForm — omitted fields fall back to the same defaults the
in-app "new event" uses (next full hour, +1h). The form is routed through
the existing single-event prefill channel (RootScreen → CalendarHost →
the create form for review), with calendarId left null so it resolves to
the last-used / first-writable calendar. No new permission is needed
(WRITE_CALENDAR is already held), and the user still explicitly saves.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/AndroidManifest.xml

@@ -112,6 +112,17 @@
                 <data android:mimeType="text/calendar" />
             </intent-filter>
 
+            <!-- Let another app or widget (e.g. the Todo Agenda widget) launch us
+                 to create a new event, the way the AOSP calendar accepts it:
+                 ACTION_INSERT on the events dir mime type, carrying the new
+                 event's fields as CalendarContract extras
+                 (MainActivity.insertFormOrNull, issue #30). -->
+            <intent-filter>
+                <action android:name="android.intent.action.INSERT" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <data android:mimeType="vnd.android.cursor.dir/event" />
+            </intent-filter>
+
             <!-- Launcher long-press shortcuts (e.g. "New event"). -->
             <meta-data
                 android:name="android.app.shortcuts"