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());
+    }
+};

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

@@ -22,6 +22,85 @@ export async function load({ fetch, params, url}) {
     // Get multiplier from URL parameters
     const multiplier = parseFloat(url.searchParams.get('multiplier') || '1');
     
+    // Handle yeast swapping from URL parameters based on toggle flags
+    // Look for parameters like y0=1, y1=1 (yeast #0 and #1 are toggled)
+    if (item.ingredients) {
+        let yeastCounter = 0;
+        
+        // Iterate through all ingredients to find yeast and apply conversions
+        for (let listIndex = 0; listIndex < item.ingredients.length; listIndex++) {
+            const list = item.ingredients[listIndex];
+            if (list.list) {
+                for (let ingredientIndex = 0; ingredientIndex < list.list.length; ingredientIndex++) {
+                    const ingredient = list.list[ingredientIndex];
+                    
+                    // Check if this is a yeast ingredient
+                    if (ingredient.name === "Frischhefe" || ingredient.name === "Trockenhefe") {
+                        // Check if this yeast should be toggled
+                        const yeastParam = `y${yeastCounter}`;
+                        const isToggled = url.searchParams.has(yeastParam);
+                        
+                        if (isToggled) {
+                            // Perform yeast conversion from original recipe data
+                            const originalName = ingredient.name;
+                            const originalAmount = parseFloat(ingredient.amount);
+                            const originalUnit = ingredient.unit;
+                            
+                            let newName: string, newAmount: string, newUnit: string;
+                            
+                            if (originalName === "Frischhefe") {
+                                // Convert fresh yeast to dry yeast
+                                newName = "Trockenhefe";
+                                
+                                if (originalUnit === "Prise") {
+                                    // "1 Prise Frischhefe" → "1 Prise Trockenhefe"
+                                    newAmount = ingredient.amount;
+                                    newUnit = "Prise";
+                                } else if (originalUnit === "g" && originalAmount === 1) {
+                                    // "1 g Frischhefe" → "1 Prise Trockenhefe"
+                                    newAmount = "1";
+                                    newUnit = "Prise";
+                                } else {
+                                    // Normal conversion: "9 g Frischhefe" → "3 g Trockenhefe" (divide by 3)
+                                    newAmount = (originalAmount / 3).toString();
+                                    newUnit = "g";
+                                }
+                            } else if (originalName === "Trockenhefe") {
+                                // Convert dry yeast to fresh yeast
+                                newName = "Frischhefe";
+                                
+                                if (originalUnit === "Prise") {
+                                    // "1 Prise Trockenhefe" → "1 g Frischhefe"
+                                    newAmount = "1";
+                                    newUnit = "g";
+                                } else {
+                                    // Normal conversion: "1 g Trockenhefe" → "3 g Frischhefe" (multiply by 3)
+                                    newAmount = (originalAmount * 3).toString();
+                                    newUnit = "g";
+                                }
+                            } else {
+                                // Fallback
+                                newName = originalName;
+                                newAmount = ingredient.amount;
+                                newUnit = originalUnit;
+                            }
+                            
+                            // Apply the conversion
+                            item.ingredients[listIndex].list[ingredientIndex] = {
+                                ...item.ingredients[listIndex].list[ingredientIndex],
+                                name: newName,
+                                amount: newAmount,
+                                unit: newUnit
+                            };
+                        }
+                        
+                        yeastCounter++;
+                    }
+                }
+            }
+        }
+    }
+    
     return {
         ...item,
         isFavorite,