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/routes/account/+page.server.ts

@@ -1,4 +1,4 @@
-import { auth, authProviders } from '$lib/server/auth';
+import { auth, authProviders } from '#lib/server/auth.js';
 import { redirect } from '@sveltejs/kit';
 import type { PageServerLoad } from './$types';
 export const load: PageServerLoad = async ({ locals, request, url }) => {

src/routes/account/+page.svelte

@@ -1,9 +1,9 @@
 <script lang="ts">
 	import { resolve } from '$app/paths';
-	import { authClient } from '$lib/auth-client';
-	import { Button } from '$lib/components/ui/button/index.js';
-	import * as Card from '$lib/components/ui/card/index.js';
-	import { Badge } from '$lib/components/ui/badge/index.js';
+	import { authClient } from '#lib/auth-client.js';
+	import { Button } from '#lib/components/ui/button/index.js';
+	import * as Card from '#lib/components/ui/card/index.js';
+	import { Badge } from '#lib/components/ui/badge/index.js';
 	import type { PageProps } from './$types';
 	let { data }: PageProps = $props();
 	let busy = $state(false);
@@ -66,11 +66,11 @@
 		<Card.Root
 			><Card.Header><Card.Title>Administration</Card.Title></Card.Header><Card.Content>
 				<div class="flex flex-wrap gap-4">
-					<a class="underline" href={resolve('/blog')}>Blog articles</a><a
+					<a class="underline" href={resolve('blog')}>Blog articles</a><a
 						class="underline"
-						href={resolve('/lists')}>Lists</a
-					><a class="underline" href={resolve('/projects/animaldex/admin')}>Animaldex moderation</a
-					><a class="underline" href={resolve('/projects/progress')}>Progress</a>
+						href={resolve('lists')}>Lists</a
+					><a class="underline" href={resolve('projects/animaldex/admin')}>Animaldex moderation</a
+					><a class="underline" href={resolve('projects/progress')}>Progress</a>
 				</div>
 			</Card.Content></Card.Root
 		>