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/animaldex/AnimalDetails.svelte

@@ -101,7 +101,7 @@
 		</dl>
 
 		<a
-			href={resolve(`/projects/animaldex/register?animalId=${animal.id}`)}
+			href={resolve(`projects/animaldex/register?animalId=${animal.id}`)}
 			class="mt-6 inline-flex items-center gap-2 rounded bg-[var(--site-text)] px-4 py-2.5 text-base font-medium text-[var(--site-bg)]"
 		>
 			i saw this animal too <ArrowRight size={16} aria-hidden="true" />