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 <[email protected]>

src/lib/components/ui/alert-dialog/alert-dialog-overlay.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 	import { AlertDialog as AlertDialogPrimitive } from 'bits-ui';
-	import { cn } from '$lib/utils.js';
+	import { cn } from '#lib/utils.js';
 
 	let {
 		ref = $bindable(null),