feat: private accounts, explore/people merge, meal-plan fixes, i18n and theme cleanup

- Private accounts: users.isPrivate hides a user from search and their
  recipes from search/trending/for-you discovery surfaces (follow-aware
  where the route already has session context, blanket exclusion where it
  doesn't); existing followers and direct links are unaffected, no
  follow-request approval flow was built (explicit scope limit)
- Merged /people into the Explore tab (tab=people query param); the old
  standalone route now redirects there
- "Get Ideas" vs "Surprise Me" were doing the same empty-prompt call;
  Surprise Me now injects a real random constraint (5-ingredient, one-pot,
  etc.), matching the existing pattern in the AI recipe-generate dialog
- Meal-plan day cells now link to their recipe (was dead text) and gained
  a one-click "mark as cooked" action
- Theme toggle is now a real three-way light/dark/system control instead
  of a binary flip
- Nutrition goals form had zero i18n wiring; fully localized now

New migration 0027 (users.is_private) generated, left unapplied like the
others. Verified with typecheck, lint, and a clean --no-cache docker build.

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

apps/web/app/api/v1/users/me/route.ts

@@ -9,6 +9,7 @@ const PatchSchema = z.object({
   locale: z.string().max(10).optional(),
   bio: z.string().max(500).optional().nullable(),
   privateBio: z.string().max(2000).optional().nullable(),
+  isPrivate: z.boolean().optional(),
 });
 
 export async function PATCH(req: Request) {