feat: note sorting with drag-and-drop custom order

- Add sort mode selector (Updated/Created/Custom) with localStorage persistence
- Sort notes client-side via derived stores based on selected mode
- Enable drag-to-reorder in Custom mode via svelte-dnd-action on NoteGrid
- Add POST /api/notes/reorder endpoint for bulk sortOrder updates
- E2E tests for sorting by updated/created and persistence across reload

Closes #12

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/routes/api/notes/reorder/+server.ts

@@ -0,0 +1,22 @@
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types.js';
+import { db } from '$lib/server/db/index.js';
+import { notes } from '$lib/server/db/schema.js';
+import { eq } from 'drizzle-orm';
+
+export const POST: RequestHandler = async ({ request }) => {
+	const body = await request.json();
+	const orders: { id: string; sortOrder: number }[] = body.orders;
+
+	if (!Array.isArray(orders)) {
+		return json({ error: 'Invalid payload' }, { status: 400 });
+	}
+
+	db.transaction((tx) => {
+		for (const { id, sortOrder } of orders) {
+			tx.update(notes).set({ sortOrder }).where(eq(notes.id, id)).run();
+		}
+	});
+
+	return json({ ok: true });
+};