feat: upgrade to SvelteKit 3 and adapter-bun

Run the sveltekit-3 migration and finish its manual tasks:

- Move kit config into the sveltekit() Vite plugin; drop svelte.config.js
- Replace $lib with #lib subpath imports
- Declare env vars in src/env.ts and import them from $app/env/{private,public}
- Consolidate param matchers into src/params.ts
- Use error(status, message), Response.json, refreshAll and goto reset/shallow
- Fall back to a full page load when goto rejects a login return path

Switch from adapter-node to adapter-bun: the build script runs Vite under
Bun (the adapter requires it) and the image starts with `bun ./build`.
Runtime env names (PORT, BODY_SIZE_LIMIT, ADDRESS_HEADER, ...) are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib/server/osu.test.ts

@@ -1,5 +1,5 @@
 import { expect, it, vi } from 'vitest';
-vi.mock('$env/dynamic/private', () => ({ env: { OSU_CLIENT_ID: '1', OSU_CLIENT_SECRET: 'test' } }));
+vi.mock('$app/env/private', () => ({ OSU_CLIENT_ID: '1', OSU_CLIENT_SECRET: 'test' }));
 import { getOsuMaps } from './osu';
 it('falls back deliberately, merges IDs, reuses OAuth and caches matching results', async () => {
 	const fetchFn = vi.fn(async (url: string | URL | Request) => {
@@ -22,14 +22,11 @@ it('falls back deliberately, merges IDs, reuses OAuth and caches matching result
 		artists: [{ name: 'Artist' }],
 		album: { name: '', images: [] }
 	};
-	const results = await Promise.all([
-		getOsuMaps(fetchFn as typeof fetch, track),
-		getOsuMaps(fetchFn as typeof fetch, track)
-	]);
+	const results = await Promise.all([getOsuMaps(fetchFn, track), getOsuMaps(fetchFn, track)]);
 	expect(results[0]).toHaveLength(1);
 	expect(results[0][0].score).toBe(100);
 	expect(results[1]).toEqual(results[0]);
-	await getOsuMaps(fetchFn as typeof fetch, track);
+	await getOsuMaps(fetchFn, track);
 	expect(fetchFn).toHaveBeenCalledTimes(3);
 	expect(fetchFn.mock.calls.filter(([url]) => String(url).includes('/oauth/token'))).toHaveLength(
 		1