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/hooks.server.ts

@@ -1,6 +1,6 @@
-import { building } from '$app/environment';
-import { auth } from '$lib/server/auth';
-import type { Handle } from '@sveltejs/kit';
+import type { Handle } from '@sveltejs/kit/hooks';
+import { building } from '$app/env';
+import { auth } from '#lib/server/auth.js';
 
 export const handle: Handle = async ({ event, resolve }) => {
 	// Match the endpoint path here: the dev proxy may terminate HTTPS before Vite.