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>

tsconfig.json

@@ -1,15 +1,8 @@
 {
-	"extends": "./.svelte-kit/tsconfig.json",
+	"extends": "$app/tsconfig",
 	"compilerOptions": {
-		"allowJs": true,
-		"checkJs": true,
-		"esModuleInterop": true,
-		"forceConsistentCasingInFileNames": true,
-		"resolveJsonModule": true,
-		"skipLibCheck": true,
 		"sourceMap": true,
-		"strict": true,
-		"moduleResolution": "bundler"
+		"strict": true
 	},
 	"exclude": [
 		"node_modules/**",
@@ -20,10 +13,6 @@
 		"src/service-worker/**/*.ts",
 		"src/service-worker.d.ts",
 		"src/service-worker/**/*.d.ts"
-	]
-	// Path aliases are handled by https://kit.svelte.dev/docs/configuration#alias
-	// except $lib which is handled by https://kit.svelte.dev/docs/configuration#files
-	//
-	// If you want to overwrite includes/excludes, make sure to copy over the relevant includes/excludes
-	// from the referenced tsconfig.json - TypeScript does not merge them in
+	],
+	"include": ["src", "vite.config.ts", "drizzle.config.ts", "playwright.config.ts", "tests"]
 }