feat: 6 new per-tier feature flags (off by default) + flag-icon locale switcher + hide BYOK when disabled (v0.74.0)

Extends the existing feature-flags system (previously 3 keys, all
default-enabled) with 6 more: recipe_import_url, recipe_import_photo,
nutrition_estimation, markdown_export, weekly_nutrition,
grocery_delivery. Each FEATURE_DEFINITIONS entry now carries its own
defaultEnabled -- the new 6 default to false, the original 3 stay
true -- so no migration/seed was needed for the "off by default"
requirement, just a per-key fallback instead of a blanket true.

Gated server-side (requireFeatureEnabledResponse, a new shared
helper avoiding six copies of the same try/catch) on: import-url,
import-photo, nutrition POST estimate, bulk markdown export, weekly
meal-plan nutrition GET, Instacart export. Gated client-side by
hiding the trigger entirely (not just disabling) on every page that
renders one: recipe detail (meal/drink pairing buttons, nutrition
panel's estimate button, markdown export), recipes list (import-URL
button, including the OS Share Target auto-import path), new-recipe
page (photo import), meal-plan page (markdown export, weekly
nutrition bar), shopping-list/collection/pantry pages (markdown
export), shopping-list page (Instacart button, now gated by both the
existing env-var check AND the tier flag).

Also: BYOK section on Settings -> AI now hidden entirely for
non-BYOK users (previously showed a locked-and-teased notice, same
inconsistency the Model Prefs fix closed yesterday). Language
switcher shows a flag icon (FlagGB/FlagFR, moved from
components/marketing to components/shared so both the logged-in
settings switcher and the logged-out marketing one can use it)
instead of plain "English"/"Français" text-only options.

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

apps/web/app/(app)/shopping-lists/[id]/page.tsx

@@ -16,6 +16,7 @@ import { getShoppingListAccess, canWriteShoppingList } from "@/lib/shopping-list
 import { ExportMarkdownButton } from "@/components/shared/export-markdown-button";
 import { shoppingListToMarkdown } from "@/lib/markdown/shopping-list";
 import { getMessages, formatMessage } from "@/lib/i18n/server";
+import { getFeatureFlagMatrix, type Tier } from "@/lib/feature-flags";
 
 type Params = { params: Promise<{ id: string }> };
 
@@ -37,7 +38,10 @@ export default async function ShoppingListPage({ params }: Params) {
   if (!list) notFound();
 
   const canEdit = canWriteShoppingList(access.role);
-  const instacartEnabled = process.env["NEXT_PUBLIC_GROCERY_PROVIDER"] === "instacart";
+  const viewerTier = (session.user as { tier?: string }).tier as Tier | undefined ?? "free";
+  const featureFlags = await getFeatureFlagMatrix();
+  const canExportMarkdown = featureFlags.markdown_export[viewerTier];
+  const instacartEnabled = process.env["NEXT_PUBLIC_GROCERY_PROVIDER"] === "instacart" && featureFlags.grocery_delivery[viewerTier];
 
   return (
     <div className="space-y-6 max-w-2xl">
@@ -63,10 +67,12 @@ export default async function ShoppingListPage({ params }: Params) {
             } />
             <TooltipContent>{m.common.print}</TooltipContent>
           </Tooltip>
-          <ExportMarkdownButton
-            markdown={shoppingListToMarkdown({ name: list.name, items: list.items })}
-            filename={list.name}
-          />
+          {canExportMarkdown && (
+            <ExportMarkdownButton
+              markdown={shoppingListToMarkdown({ name: list.name, items: list.items })}
+              filename={list.name}
+            />
+          )}
           {access.role === "owner" && (
             <ShoppingListActionsMenu listId={id} name={list.name} redirectAfterDeleteTo="/shopping-lists" />
           )}