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/app.d.ts

@@ -6,12 +6,12 @@ declare global {
 	namespace App {
 		// interface Error {}
 		interface Locals {
-			user: typeof import('$lib/server/auth').auth.$Infer.Session.user | null;
-			session: typeof import('$lib/server/auth').auth.$Infer.Session.session | null;
+			user: typeof import('#lib/server/auth.js').auth.$Infer.Session.user | null;
+			session: typeof import('#lib/server/auth.js').auth.$Infer.Session.session | null;
 		}
 		// interface PageData {}
 		interface PageState {
-			animal?: import('$lib/animaldex/AnimalDetails.svelte').AnimalDetail;
+			animal?: import('#lib/animaldex/AnimalDetails.svelte').AnimalDetail;
 		}
 		// interface Platform {}
 	}