feat: anonymous read-only public link + QR code for meal plans (v0.68.0)

Mirrors shopping lists' isPublic pattern but read-only only -- no
publicEditable equivalent, since anonymous editing of someone's
weekly meal schedule isn't a real use case the way collaborative
shopping-list editing is. New PATCH /api/v1/meal-plans/{weekStart}
toggles it (lazily creates the week's plan row if missing, same
pattern as the existing POST). Public page at /m/[id] renders the
week grouped by day, linking through to public/unlisted recipes and
just showing the title otherwise.

ShareMealPlanButton gained the same public-link toggle UI as its
shopping-list sibling, plus a QR code (generated client-side via the
already-installed `qrcode` package) for the link once enabled --
handing someone a printed or screenshotted plan doesn't require
typing a URL.

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

apps/web/messages/en.json

@@ -982,6 +982,9 @@
     "print": "Print",
     "shareTitle": "Share this week's plan",
     "shareDescription": "Invite household members to view or edit this week's meal plan.",
+    "publicLinkReadOnlyDescription": "Anyone with the link can view this week's plan (read-only) — no account needed.",
+    "qrCodeAlt": "QR code linking to this week's public meal plan",
+    "noEntriesPublic": "Nothing planned for this week yet.",
     "sharedWithYou": "Shared with you",
     "sharedPlanOf": "{name}'s plan",
     "weekOf": "Week of {date}",