feat: add comprehensive filter UI with chip-based dropdowns

Add advanced filtering with category, tags (multi-select), icon, season,
and favorites filters. All filters use consistent chip-based dropdown UI
with type-to-search functionality.

New Components:
- TagChip.svelte: Reusable chip component with selected/removable states
- CategoryFilter.svelte: Single-select category with chip dropdown
- TagFilter.svelte: Multi-select tags with AND logic and chip dropdown
- IconFilter.svelte: Single-select emoji icon with chip dropdown
- SeasonFilter.svelte: Multi-select months with chip dropdown
- FavoritesFilter.svelte: Toggle for favorites-only filtering
- FilterPanel.svelte: Container with responsive layout and mobile toggle

Search Component:
- Integrated FilterPanel with all filter types
- Added applyNonTextFilters() for category/tags/icon/season/favorites
- Implemented favorites filter logic (recipe.isFavorite check)
- Made tags/icons reload reactively when language changes with $effect
- Updated buildSearchUrl() for comma-separated array parameters
- Passed categories and isLoggedIn props to enable all filters

Server API:
- Both /api/rezepte/search and /api/recipes/search support:
  - Multi-tag AND logic using MongoDB $all operator
  - Multi-season filtering using MongoDB $in operator
  - Backwards compatible with single tag/season parameters
- Updated search page server load to parse tag/season arrays

UI/UX:
- Filters display inline on wide screens with 2rem gap
- Mobile: collapsible with subtle toggle button and slide-down animation
- Chip-based dropdowns appear on focus with filtering as you type
- Selected items display as removable chips below inputs (no background)
- Centered labels on desktop, left-aligned on mobile
- Reduced vertical spacing on mobile (0.3rem gap)
- Max-width constraints: 500px for filters, 600px for panel on mobile
- Consistent naming: "Tags" and "Icon" instead of German translations

src/routes/[recipeLang=recipeLang]/+page.svelte

@@ -101,7 +101,7 @@ h1{
 <h1>{labels.title}</h1>
 <p class=subheading>{labels.subheading}</p>
 
-<Search lang={data.lang} recipes={data.all_brief} onSearchResults={handleSearchResults}></Search>
+<Search lang={data.lang} recipes={data.all_brief} categories={categories} isLoggedIn={!!data.session?.user} onSearchResults={handleSearchResults}></Search>
 
 {#if seasonRecipes.length > 0}
 	<LazyCategory title={labels.inSeason} eager={true}>