Implement progressive enhancement for yeast swapper with state persistence

- Add server-side form handling for yeast swapping without JavaScript
- Implement toggle-based URL parameter system (y0=1, y1=1) for clean URLs
- Add server action to toggle yeast flags and preserve all URL state
- Update multiplier forms to preserve yeast toggle states across submissions
- Calculate yeast conversions server-side from original recipe data
- Fix {{multiplier}} placeholder replacement to handle non-numeric amounts
- Enable multiple independent yeast swappers with full state preservation
- Maintain perfect progressive enhancement: works with and without JS

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/routes/rezepte/[name]/+page.server.ts

@@ -0,0 +1,29 @@
+import { redirect } from '@sveltejs/kit';
+
+export const actions = {
+    swapYeast: async ({ request, url }) => {
+        const formData = await request.formData();
+        const yeastId = parseInt(formData.get('yeastId') as string);
+
+        // Build new URL
+        const newUrl = new URL(url.pathname, url.origin);
+        
+        // Restore all parameters from the form data (they were submitted as currentParam_*)
+        for (const [key, value] of formData.entries()) {
+            if (key.startsWith('currentParam_')) {
+                const paramName = key.substring('currentParam_'.length);
+                newUrl.searchParams.set(paramName, value as string);
+            }
+        }
+        
+        // Toggle the yeast flag - if it exists, remove it; if not, add it
+        const yeastParam = `y${yeastId}`;
+        if (newUrl.searchParams.has(yeastParam)) {
+            newUrl.searchParams.delete(yeastParam);
+        } else {
+            newUrl.searchParams.set(yeastParam, '1');
+        }
+        
+        throw redirect(303, newUrl.toString());
+    }
+};