test(calendar): grid layout, week ordering, and day-cell coverage

- Add isToday test cases to CalendarDay.test.tsx
  * test --today class when isToday=true
  * test --today class not applied when isToday=false
- Create Calendar.test.tsx with full page coverage
  * Test 7 weekday headers in Monday-first Swedish order
  * Verify correct number of day cells (multiple of 7)
  * Test Sunday-starting month renders correctly (March 2026)
  * Test modal opens on current-month day click
  * Test modal doesn't open on disabled day click
  * Test current month title display
  * Test prev/next month navigation
  * Test loading state display
- Mocks useWorkspace, usePlannedDays, Layout, LoadingSpinner, PlannedDayModal
- All 19 tests pass; no coverage regression

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

frontend/src/components/common/CalendarDay.test.tsx

@@ -93,4 +93,20 @@ describe('CalendarDay', () => {
     const events = queryByTestId('calendar-day-events')
     expect(events).not.toBeInTheDocument()
   })
+
+  it('applies today class when isToday is true', () => {
+    const { getByTestId } = render(
+      <CalendarDay date="2026-06-15" isToday={true} />
+    )
+    const day = getByTestId('calendar-day')
+    expect(day).toHaveClass('calendar-day--today')
+  })
+
+  it('does not apply today class when isToday is false', () => {
+    const { getByTestId } = render(
+      <CalendarDay date="2026-06-15" isToday={false} />
+    )
+    const day = getByTestId('calendar-day')
+    expect(day).not.toHaveClass('calendar-day--today')
+  })
 })