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/+layout.svelte

@@ -1,15 +1,15 @@
 <script lang="ts">
 	import '../app.css';
 	import { onMount } from 'svelte';
-	import { assets, resolve } from '$app/paths';
-	import { dev } from '$app/environment';
+	import { resolve, asset } from '$app/paths';
+	import { dev } from '$app/env';
 	import { page } from '$app/state';
-	import { getSeo } from '$lib/seo';
-	import CopyButton from '$lib/components/CopyButton.svelte';
-	import ProjectIcon from '$lib/components/ProjectIcon.svelte';
-	import { projects } from '$lib/projects';
-	import { Button, buttonVariants } from '$lib/components/ui/button/index.js';
-	import * as Sheet from '$lib/components/ui/sheet/index.js';
+	import { getSeo } from '#lib/seo.js';
+	import CopyButton from '#lib/components/CopyButton.svelte';
+	import ProjectIcon from '#lib/components/ProjectIcon.svelte';
+	import { projects } from '#lib/projects.js';
+	import { Button, buttonVariants } from '#lib/components/ui/button/index.js';
+	import * as Sheet from '#lib/components/ui/sheet/index.js';
 	import ArrowRight from 'lucide-svelte/icons/arrow-right';
 	import ChevronDown from 'lucide-svelte/icons/chevron-down';
 	import ExternalLink from 'lucide-svelte/icons/external-link';
@@ -179,7 +179,7 @@
 			>
 				<span class="h-7 w-7 overflow-hidden rounded-full bg-black"
 					><img
-						src="{assets}/favicon-32x32.png"
+						src={asset('favicon-32x32.png')}
 						alt=""
 						width="32"
 						height="32"
@@ -199,7 +199,7 @@
 							onpointerleave={handleProjectsPointerLeave}
 						>
 							<Button
-								href={resolve('/projects')}
+								href={resolve('projects')}
 								variant={current(item.href) ? 'secondary' : 'ghost'}
 								size="sm"
 								class="cursor-pointer gap-1"
@@ -217,7 +217,7 @@
 									aria-label="Projects"
 								>
 									<a
-										href={resolve('/projects')}
+										href={resolve('projects')}
 										class="flex items-center rounded-sm px-2 py-1.5 text-sm font-medium outline-none hover:bg-accent hover:text-accent-foreground focus-visible:bg-accent"
 										>all projects <ArrowRight size={14} class="ml-auto" /></a
 									>
@@ -266,13 +266,13 @@
 					title={`Theme: ${theme}`}>{@render themeIcon()}</Button
 				>
 				{#if data.user}
-					<Button href={resolve('/account')} variant="outline" size="sm" class="max-w-36 truncate"
+					<Button href={resolve('account')} variant="outline" size="sm" class="max-w-36 truncate"
 						>{data.user.name}</Button
 					>
 				{:else}
 					<Button
 						href={resolve(
-							`/login?returnTo=${encodeURIComponent(page.url.pathname + page.url.search)}`
+							`login?returnTo=${encodeURIComponent(page.url.pathname + page.url.search)}`
 						)}
 						variant="outline"
 						size="sm">sign in</Button
@@ -322,20 +322,18 @@
 							aria-label={`Theme: ${theme}. Switch mode`}>{@render themeIcon()}</Button
 						>
 						{#if data.user}
-							<Button
-								href={resolve('/account')}
-								variant="outline"
-								onclick={() => (menuOpen = false)}>account</Button
+							<Button href={resolve('account')} variant="outline" onclick={() => (menuOpen = false)}
+								>account</Button
 							>
 							<form method="POST" action="/logout">
 								<Button type="submit" variant="outline">sign out</Button>
 							</form>
 						{:else}
-							<Button href={resolve('/login')} variant="outline" onclick={() => (menuOpen = false)}
+							<Button href={resolve('login')} variant="outline" onclick={() => (menuOpen = false)}
 								>sign in</Button
 							>
 							<Button
-								href={resolve('/register')}
+								href={resolve('register')}
 								variant="outline"
 								onclick={() => (menuOpen = false)}>register</Button
 							>