Implement MiniCalendar dashboard widget (P2.14)

Complete the MiniCalendar component with:
- Full calendar grid showing all days of the month
- Phase colors applied to each day
- Today highlighting with ring indicator
- Navigation buttons (prev/next month, Today)
- Compact phase legend
- Integration into dashboard page (shows when lastPeriodDate exists)

Adds 23 new tests for the MiniCalendar component covering:
- Calendar grid rendering
- Phase color application
- Navigation functionality
- Cycle rollover handling
- Custom year/month props

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/page.test.tsx

@@ -564,7 +564,8 @@ describe("Dashboard", () => {
       render(<Dashboard />);
 
       await waitFor(() => {
-        expect(screen.getByText(/follicular/i)).toBeInTheDocument();
+        // Check for phase in the cycle info header (uppercase, with Day X prefix)
+        expect(screen.getByText(/Day 12 · FOLLICULAR/)).toBeInTheDocument();
       });
     });
   });