refactor(planner): replace hardcoded values with design tokens

- border-radius: 10px → var(--radius-lg) in both tile components
- opacity: 0.42 → var(--opacity-dimmed) in DesktopDayTile
- var(--yellow) → var(--color-ring-today) for today ring and date circle

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/planner/EmptyDayTile.svelte

@@ -31,7 +31,7 @@
 	data-testid="empty-day-tile"
 	role="group"
 	class="h-full flex flex-col overflow-hidden"
-	style="border: 1.5px dashed var(--color-border); border-radius: 10px; background: var(--color-surface); box-shadow: 0 1px 3px rgba(28,28,24,.06);"
+	style="border: 1.5px dashed var(--color-border); border-radius: var(--radius-lg); background: var(--color-surface); box-shadow: 0 1px 3px rgba(28,28,24,.06);"
 >
 	<!-- Day header -->
 	<div style="display: flex; align-items: center; justify-content: space-between; padding: 7px 8px 0; flex-shrink: 0;">