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>

package.json

@@ -4,7 +4,7 @@
 	"private": true,
 	"scripts": {
 		"dev": "vite dev",
-		"build": "vite build",
+		"build": "bun --bun vite build",
 		"preview": "vite preview",
 		"test": "npm run test:integration && npm run test:unit",
 		"check": "svelte-kit sync && svelte-check --tsgo --tsconfig ./tsconfig.json",
@@ -49,8 +49,9 @@
 		"@internationalized/date": "^3.12.0",
 		"@lucide/svelte": "^1.48.0",
 		"@playwright/test": "^1.62.1",
+		"@sveltejs/adapter-bun": "^1.0.0",
 		"@sveltejs/enhanced-img": "^0.11.0",
-		"@sveltejs/kit": "^2.70.3",
+		"@sveltejs/kit": "^3.0.0",
 		"@sveltejs/vite-plugin-svelte": "^7.3.0",
 		"@tailwindcss/typography": "^0.5.20",
 		"@tailwindcss/vite": "^4.3.3",
@@ -73,7 +74,7 @@
 		"prettier": "3.9.6",
 		"prettier-plugin-svelte": "^4.1.1",
 		"shadcn-svelte": "^1.7.0",
-		"svelte": "^5.57.0",
+		"svelte": "^5.57.1",
 		"svelte-check": "4.7.6",
 		"tailwind-merge": "^3.6.0",
 		"tailwind-variants": "^3.3.1",
@@ -91,7 +92,6 @@
 		"@aws-sdk/client-s3": "^3.1121.0",
 		"@aws-sdk/s3-request-presigner": "^3.1121.0",
 		"@maptiler/sdk": "^4.1.0",
-		"@sveltejs/adapter-node": "^5.5.7",
 		"ai": "^7.0.85",
 		"d3-geo": "^3.1.1",
 		"drizzle-orm": "^1.0.0-rc.4",
@@ -104,5 +104,9 @@
 		"topojson-client": "^3.1.0",
 		"world-atlas": "^2.0.2",
 		"zod": "^4.5.4"
+	},
+	"imports": {
+		"#lib": "./src/lib/index.js",
+		"#lib/*": "./src/lib/*"
 	}
 }