Feature 3: Read-only Google Calendar panel

- CalendarEvent model with summary, start, end, location
- New scope calendar.readonly in SCOPES
- fetch_upcoming_events() in ApiClient (15 events, next 7 days)
- Focus::Calendar variant, Tab cycle includes Calendar
- Body layout: left column split into Tasks (flex) + Calendar (8)
- render_calendar_panel with day headers and scroll
- refresh_calendar() called on initial sync and Ctrl+R
- Up/Down scroll Calendar panel when focused

src/domain/models.rs

@@ -46,3 +46,11 @@ pub struct SyncQueueItem {
 }
 
 pub const MAX_SYNC_RETRIES: i32 = 3;
+
+#[derive(Debug, Clone)]
+pub struct CalendarEvent {
+    pub summary: String,
+    pub start: Option<chrono::NaiveDateTime>,
+    pub end: Option<chrono::NaiveDateTime>,
+    pub location: Option<String>,
+}