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/game-tracker/game-art.ts

@@ -1,5 +1,6 @@
 import releases from '../../../data/pokemon-releases.json';
-import { base } from '$app/paths';
+import { asset } from '$app/paths';
+import type { AssetPath } from '$app/types';
 
 const dates: Record<string, string> = releases.dates;
 export function pokemonRelease(slug: string | null) {
@@ -11,5 +12,5 @@ export function chronologicalPokemon<T extends { slug: string | null; id: number
 	);
 }
 export function pokemonImage(number: number, kind: 'sprite' | 'shiny' | 'artwork' = 'sprite') {
-	return `${base}/game-art/pokemon/${kind}/${number}.webp`;
+	return asset(`game-art/pokemon/${kind}/${number}.webp` as AssetPath);
 }