refactor: unify recipe routes into [recipeLang] slug with full bilingual support

Consolidate /rezepte and /recipes routes into single [recipeLang] structure to eliminate code duplication. All pages now use conditional API routing and reactive labels based on language parameter.

- Merge duplicate route structures into /[recipeLang] with 404 for invalid slugs
- Add English API endpoints for search, favorites, tags, and categories
- Implement language dropdown in header with localStorage persistence
- Convert all pages to use Svelte 5 runes (, , )
- Add German-only redirects (301) for add/edit pages
- Make all view pages (list, detail, filters, search, favorites) fully bilingual
- Remove floating language switcher in favor of header dropdown

src/routes/[recipeLang]/tips-and-tricks/Converter.svelte

@@ -0,0 +1,68 @@
+<script>
+	class HefeConverter {
+		constructor(trockenhefe = 1) {
+			this._trockenhefe = trockenhefe;
+			this._frischhefe = this._trockenhefe * 3;
+		}
+
+		get trockenhefe() {
+			return Math.round(this._trockenhefe * 100) / 100 + "g";
+		}
+
+		set trockenhefe(value) {
+			this._trockenhefe = value.replace(/\D/g, '');
+			this._frischhefe = this._trockenhefe * 3;
+		}
+
+		get frischhefe() {
+			return this._frischhefe+"g";
+		}
+
+		set frischhefe(value) {
+			this._frischhefe = value.replace(/\D/g, '');
+			this._trockenhefe = this._frischhefe / 3;
+		}
+	}
+
+	const hefeConverter = new HefeConverter();
+
+</script>
+<style>
+	.converter_container {
+		width: fit-content;
+		display: flex;
+		flex-direction: row;
+		justify-content: center;
+		gap: 2rem;
+		background-color: var(--blue);
+		padding: 2rem;
+		margin-inline: auto;
+		align-items: center;
+	}
+	input {
+		width: 5rem;
+		height: 2rem;
+		font-size: 1rem;
+		text-align: center;
+	}
+	.flex_column {
+		display: flex;
+		flex-direction: column;
+	}
+	label {
+		margin-bottom: 0.25rem;
+	}
+</style>
+<div class=converter_container>
+	<div class="flex_column">
+		<label for="trockenhefe">Trockenhefe</label>
+		<input type="text" bind:value={hefeConverter.trockenhefe} min="0" />
+	</div>
+	<div>
+		=
+	</div>
+	<div class="flex_column">
+		<label for="frischhefe">Frischhefe</label>
+		<input type="text" bind:value={hefeConverter.frischhefe} min="0"/>
+	</div>
+</div>