feat: add organization context model

Introduce organizations as the top-level workspace model with address and bank-detail fields.

Store the active organization on Better Auth sessions and load organization context for dashboard routes.

Add the migration that creates organizations and backfills existing records to the default organization.

src/routes/dashboard/+layout.server.ts

@@ -0,0 +1,11 @@
+import { redirect } from '@sveltejs/kit';
+import { loadOrganizationContext } from '$lib/server/organizations';
+import type { LayoutServerLoad } from './$types';
+
+export const load: LayoutServerLoad = async ({ locals }) => {
+	if (!locals.user) {
+		redirect(303, '/login');
+	}
+
+	return loadOrganizationContext(locals);
+};