feat: export meal plan to a calendar (.ics) file

Hand-rolled a minimal RFC 5545 writer (lib/ics.ts) rather than adding
a dependency for what's a handful of VEVENTs. Meal slots have no
stored time-of-day, so each mealType maps to a conventional wall-clock
time (dinner 7pm, etc.) written as floating local time, not pinned to
a timezone. Authenticated-only download for now, not a subscribe URL
— that would need a new unguessable-link mechanism on meal plans,
which don't have one today (unlike shopping lists' isPublic).

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

apps/web/app/(app)/meal-plan/page.tsx

@@ -1,7 +1,7 @@
 import type { Metadata } from "next";
 import { headers } from "next/headers";
 import Link from "next/link";
-import { ChevronLeft, ChevronRight, ShoppingCart, Printer } from "lucide-react";
+import { ChevronLeft, ChevronRight, ShoppingCart, Printer, CalendarDays } from "lucide-react";
 import { auth } from "@/lib/auth/server";
 import { db, mealPlans, mealPlanMembers, recipes, userNutritionGoals, eq, and, desc } from "@epicure/db";
 import { buttonVariants } from "@/components/ui/button";
@@ -133,6 +133,10 @@ export default async function MealPlanPage({
             markdown={mealPlanToMarkdown({ label, entries })}
             filename={`meal-plan-${weekStart}`}
           />
+          <a href={`/api/v1/meal-plans/${weekStart}/export/ics`} className={cn(buttonVariants({ variant: "outline", size: "sm" }))}>
+            <CalendarDays className="h-4 w-4" />
+            {msgs.mealPlan.exportCalendar}
+          </a>
           <Link href={`/meal-plan?week=${prevWeek}`} className={cn(buttonVariants({ variant: "outline", size: "sm" }))}>
             <ChevronLeft className="h-4 w-4" />
           </Link>