fix: open existing events from external VIEW intents (#48)

Follow-up to #30. v2.14.0 handles ACTION_INSERT (the widget "+"), but
tapping an existing event in a third-party widget (e.g. Todo Agenda) never
offered Calendula, because nothing handled ACTION_VIEW on
content://com.android.calendar/events/<id>.

- Manifest: add a VIEW intent-filter matched by the provider's item MIME
  type (vnd.android.cursor.item/event), mirroring AOSP Calendar and the
  sibling INSERT dir/event filter. A content: VIEW intent carries the
  resolved type, so a path-only filter wouldn't match it.
- MainActivity.viewEventKeyOrNull: parse the events URI into the existing
  occurrence detail-key channel (the one reminder taps use). Occurrence
  times ride as EXTRA_EVENT_BEGIN_TIME/END_TIME when the launcher supplies
  them; a bare URI omits them.
- EventDetailViewModel: a NO_OCCURRENCE_TIME sentinel makes loadDetail keep
  the event row's own DTSTART/DTEND for a bare URI instead of overriding to
  the epoch (would otherwise render at 1970).

Needs on-device verification (intent-filter matching + the widget's actual
extras).

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

app/src/main/AndroidManifest.xml

@@ -123,6 +123,22 @@
                 <data android:mimeType="vnd.android.cursor.dir/event" />
             </intent-filter>
 
+            <!-- Open an existing event another app/widget points at (e.g. tapping
+                 an event in the Todo Agenda widget): ACTION_VIEW on
+                 content://com.android.calendar/events/<id>, the way AOSP fires it.
+                 Matched by the provider's item MIME type, not the path — a
+                 content: VIEW intent carries the resolved type
+                 (vnd.android.cursor.item/event) and a path-only filter wouldn't
+                 match it. The occurrence's times ride as EXTRA_EVENT_BEGIN_TIME /
+                 EXTRA_EVENT_END_TIME when the launcher supplies them
+                 (MainActivity.viewEventKeyOrNull, issue #48). -->
+            <intent-filter>
+                <action android:name="android.intent.action.VIEW" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+                <data android:mimeType="vnd.android.cursor.item/event" />
+            </intent-filter>
+
             <!-- Launcher long-press shortcuts (e.g. "New event"). -->
             <meta-data
                 android:name="android.app.shortcuts"