refactor: add proper types for all database insert operations

- Add missing Select and Insert types for Auth.js tables (Account, Session, VerificationToken)
- Update all insert operations to use typed New* variables:
  - NewUser for user signup
  - NewItem for adding wishlist items
  - NewSavedWishlist for saving wishlists
- Improves type safety and catches insert errors at compile time

src/routes/signup/+page.server.ts

@@ -1,7 +1,7 @@
 import { fail, redirect } from '@sveltejs/kit';
 import type { Actions, PageServerLoad } from './$types';
 import { db } from '$lib/server/db';
-import { users } from '$lib/db/schema';
+import { users, type NewUser } from '$lib/db/schema';
 import { eq } from 'drizzle-orm';
 import bcrypt from 'bcrypt';
 import { env } from '$env/dynamic/private';
@@ -64,11 +64,12 @@ export const actions: Actions = {
 
     const hashedPassword = await bcrypt.hash(password, 14);
 
-    await db.insert(users).values({
+    const newUser: NewUser = {
       name: sanitizedName,
       username: sanitizedUsername,
       password: hashedPassword
-    });
+    };
+    await db.insert(users).values(newUser);
 
     throw redirect(303, '/signin?registered=true');
   }