fix(17-03): lift FAB above BottomTabBar and reserve phone content padding

- CalendarShell.tsx: FAB bottom changed from var(--space-6) to
  calc(var(--bottom-chrome-h) + var(--space-6)) — sits 24px above bar
- App.tsx: contentStyle gains phone-only paddingBottom: var(--bottom-chrome-h)
  via spread pattern ...(phone ? {...} : {}) — desktop unchanged

apps/pwa/src/components/CalendarShell.tsx

@@ -467,7 +467,7 @@ export function CalendarShell() {
           onClick={() => setEventForm(true, 'create')}
           style={{
             position: 'fixed',
-            bottom: 'var(--space-6)',
+            bottom: 'calc(var(--bottom-chrome-h) + var(--space-6))',
             right: 'var(--space-6)',
             width: '56px',
             height: '56px',