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)/collections/[id]/page.tsx

@@ -18,6 +18,7 @@ import { buttonVariants } from "@/components/ui/button";
 import { Tooltip, TooltipContent, TooltipProvider, TooltipTrigger } from "@/components/ui/tooltip";
 import { cn } from "@/lib/utils";
 import { ExportMarkdownButton } from "@/components/shared/export-markdown-button";
+import { getFeatureFlagMatrix, type Tier } from "@/lib/feature-flags";
 import { EmptyState } from "@/components/shared/empty-state";
 import { collectionToMarkdown } from "@/lib/markdown/collection";
 import { getMessages } from "@/lib/i18n/server";
@@ -31,6 +32,8 @@ export default async function CollectionPage({ params }: Params) {
   const session = await auth.api.getSession({ headers: await headers() });
   if (!session) return null;
   const m = getMessages((session.user as { locale?: string }).locale);
+  const viewerTier = (session.user as { tier?: string }).tier as Tier | undefined ?? "free";
+  const canExportMarkdown = (await getFeatureFlagMatrix()).markdown_export[viewerTier];
 
   const col = await db.query.collections.findFirst({
     where: and(eq(collections.id, id), collectionVisibleToViewer(session.user.id)),
@@ -82,14 +85,14 @@ export default async function CollectionPage({ params }: Params) {
                   } />
                   <TooltipContent>{m.collections.exportPdf}</TooltipContent>
                 </Tooltip>
-                <ExportMarkdownButton
+                {canExportMarkdown && <ExportMarkdownButton
                   markdown={collectionToMarkdown({
                     name: col.name,
                     description: col.description,
                     recipes: recipeList,
                   })}
                   filename={col.name}
-                />
+                />}
               </>
             )}
             {isOwner && <GenerateMealDialog collectionId={id} />}