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/lib/game-tracker/PlayerRankBadge.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-	import { base } from '$app/paths';
+	import { resolve } from '$app/paths';
 	import ChevronsUp from 'lucide-svelte/icons/chevrons-up';
 	import Award from 'lucide-svelte/icons/award';
 	let {
@@ -34,7 +34,9 @@
 	<span class="stat">
 		{#if game === 'black-ops-2' && prestige > 0 && prestige <= 10}
 			<img
-				src={`${base}/game-art/call-of-duty/ranks/black-ops-2-prestige-${master ? 11 : prestige}.webp`}
+				src={resolve(
+					`game-art/call-of-duty/ranks/black-ops-2-prestige-${master ? 11 : prestige}.webp`
+				)}
 				alt={master ? 'Prestige Master emblem' : `Prestige ${prestige} emblem`}
 				width="48"
 				height="48"