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/routes/projects/game-tracker/credits/+page.svelte

@@ -7,7 +7,7 @@
 <!-- Download manifest links point to reviewed external image sources. -->
 <!-- eslint-disable svelte/no-navigation-without-resolve -->
 <div class="mx-auto max-w-4xl px-4 py-10 sm:px-6">
-	<a href={resolve('/projects/game-tracker')} class="text-sm text-muted-foreground hover:underline"
+	<a href={resolve('projects/game-tracker')} class="text-sm text-muted-foreground hover:underline"
 		>← Game tracker</a
 	>
 	<h1 class="mt-6 text-3xl font-semibold">Artwork credits</h1>