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/+page.svelte

@@ -1,10 +1,10 @@
 <script lang="ts">
 	import { resolve } from '$app/paths';
-	import portrait from '$lib/assets/stan.jpg?url';
-	import ProgressBar from '$lib/components/ProgressBar.svelte';
-	import ProjectIcon from '$lib/components/ProjectIcon.svelte';
-	import * as Card from '$lib/components/ui/card/index.js';
-	import { projects } from '$lib/projects';
+	import portrait from '#lib/assets/stan.jpg?url';
+	import ProgressBar from '#lib/components/ProgressBar.svelte';
+	import ProjectIcon from '#lib/components/ProjectIcon.svelte';
+	import * as Card from '#lib/components/ui/card/index.js';
+	import { projects } from '#lib/projects.js';
 	import ArrowRight from 'lucide-svelte/icons/arrow-right';
 	import ExternalLink from 'lucide-svelte/icons/external-link';
 	import Mail from 'lucide-svelte/icons/mail';
@@ -99,7 +99,7 @@
 	<section aria-labelledby="projects-heading" class="border-t border-[var(--site-border)] pt-8">
 		<h2 id="projects-heading" class="mb-4 text-2xl font-semibold tracking-tight">
 			<a
-				href={resolve('/projects')}
+				href={resolve('projects')}
 				class="inline-flex items-center gap-1 hover:text-[var(--site-muted)]"
 				>projects <ArrowRight size={20} aria-hidden="true" /></a
 			>
@@ -167,7 +167,7 @@
 	<section aria-labelledby="lists-heading" class="pt-14 sm:pt-20">
 		<h2 id="lists-heading" class="mb-4 text-2xl font-semibold tracking-tight">
 			<a
-				href={resolve('/lists')}
+				href={resolve('lists')}
 				class="inline-flex items-center gap-1 hover:text-[var(--site-muted)]"
 				>lists <ArrowRight size={20} aria-hidden="true" /></a
 			>