feat: localStorage-backed student CRUD with zod schemas

Split the app into the three pages the navbar promises, each with one job:

- / (home)   -> read-only roster, no mutation controls
- /add       -> create only, redirects to / on success
- /edit      -> update + delete only (picker, plus /edit?id=<uuid> deep links)

Data layer (src/lib/students.ts) is the single source of truth:
- zod studentDraftSchema validates form input (trims name, lowercases email,
  coerces yearLevel to an int 1-5, restricts course to a known list)
- studentSchema = draft + id + createdAt, so types come from z.infer
- versioned key it3206.students.v1, validated on read *and* write; corrupt or
  missing data degrades to an empty list instead of throwing
- typed CRUD helpers; useSyncExternalStore hook with a stable snapshot cache,
  a window event and the storage event so every island and other tabs re-render

Footer "clear local storage" is now wired to clearStudents() (confirm dialog,
label feedback) instead of being inert, and the layout switched to min-h-svh +
an in-flow footer so it no longer overlaps long lists. Footer padding clears the
Astro dev toolbar, which was covering the link in dev.

Also adds zod, plus @astrojs/check + typescript@^6 as dev deps so astro check
runs (TS 7 does not expose the API astro check needs yet).

src/pages/404.astro

@@ -0,0 +1,16 @@
+---
+import MainLayout from "../layouts/MainLayout.astro";
+---
+
+<MainLayout title="Not found">
+    <div class="flex flex-col items-center gap-3 text-center">
+        <p class="text-xs font-medium tracking-[0.3em] text-white/40 uppercase">404</p>
+        <h1 class="text-2xl font-semibold tracking-tight">Page not found</h1>
+        <p class="text-sm text-white/50">
+            That route does not exist. The list is over here.
+        </p>
+        <a href="/" class="mt-2 underline underline-offset-4 text-white/70 hover:text-white transition-colors">
+            back to the list
+        </a>
+    </div>
+</MainLayout>