feat: implement remaining TODO.md feature ideas + fix mobile headers

Implements the six previously-unscoped feature ideas plus a mobile
layout fix reported via screenshot:

- Mobile: Recipes/Collections/Pantry/Meal Plan/Shopping Lists headers
  now stack and wrap instead of clipping buttons on narrow viewports.
- Recipe diff/compare view: word/list diff against any past version,
  next to Restore in version history.
- Shared meal plans & shopping lists: new shoppingListMembers/
  mealPlanMembers tables (viewer/editor roles, mirrors
  collectionMembers), share dialogs, membership-checked routes.
- PDF cookbook export: /print/collection/[id] renders a whole
  collection with page breaks, using the existing print-CSS pattern
  instead of adding a PDF rendering dependency.
- Grocery delivery handoff: shopping lists can copy-as-text (works
  today) or send to Instacart once INSTACART_API_KEY is configured
  (stub adapter — real API needs a partner agreement).
- Personalized "For You" feed tab: ranks public recipes by tag/
  dietary overlap with the user's favorited/highly-rated history.
- PWA: added manifest.json + icons on top of the existing service
  worker so the app is installable; cook-mode pages were already
  cached for offline use.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/app/(app)/recipes/[id]/page.tsx

@@ -149,7 +149,23 @@ export default async function RecipePage({ params }: Params) {
               }))}
             />
             <PrintButton recipeId={id} />
-            <VersionHistoryButton recipeId={id} />
+            <VersionHistoryButton
+              recipeId={id}
+              currentSnapshot={{
+                title: recipe.title,
+                description: recipe.description,
+                ingredients: recipe.ingredients.map((ing) => ({
+                  rawName: ing.rawName,
+                  quantity: ing.quantity,
+                  unit: ing.unit,
+                  note: ing.note,
+                })),
+                steps: recipe.steps.map((s) => ({
+                  instruction: s.instruction,
+                  timerSeconds: s.timerSeconds,
+                })),
+              }}
+            />
             <Tooltip>
               <TooltipTrigger render={
                 <Link href={`/recipes/${id}/edit`} className={cn(buttonVariants({ variant: "ghost", size: "icon" }))}>