fix(frontend): address all PR review concerns

- Fix 7px → 11px font-size on section headers in RecipePicker
- Extract shared slotActions.ts with UUID validation for planId/slotId/recipeId
- Load full recipe list in planner page load (was placeholder current-week slots)
- Update planner/+page.svelte to pass data.recipes as allRecipes to RecipePicker
- Update planner and recipes page.server.ts to use shared slot action helpers
- Fix planner page.server tests: add recipes mock for parallel GET load
- Update action tests to use valid UUIDs (were 'plan-1'/'r1' style strings)
- Add validation-path tests for blank/invalid input on all slot actions
- Add tests for recipes/+server.ts GET endpoint (DayPicker week navigation)

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

frontend/src/lib/planner/RecipePicker.svelte

@@ -73,7 +73,7 @@
 	<!-- Empfohlen section -->
 	{#if suggestions.length > 0}
 		<div
-			style="font-size: 7px; font-weight: 500; letter-spacing: .08em; text-transform: uppercase; color: var(--color-text-muted); padding: 5px 12px 3px; background: var(--color-subtle);"
+			style="font-size: 11px; font-weight: 500; letter-spacing: .08em; text-transform: uppercase; color: var(--color-text-muted); padding: 5px 12px 3px; background: var(--color-subtle);"
 		>
 			Empfohlen · Beste Abwechslung
 		</div>
@@ -127,7 +127,7 @@
 
 	<!-- Alle Rezepte section -->
 	<div
-		style="font-size: 7px; font-weight: 500; letter-spacing: .08em; text-transform: uppercase; color: var(--color-text-muted); padding: 5px 12px 3px; background: var(--color-subtle);"
+		style="font-size: 11px; font-weight: 500; letter-spacing: .08em; text-transform: uppercase; color: var(--color-text-muted); padding: 5px 12px 3px; background: var(--color-subtle);"
 	>
 		Alle Rezepte
 	</div>