chore: migrate to v3, Doppler secrets, clean up legacy code

- Remove all pre-v3 code: scraper, ui-v2, backend v1, ios v1+v2, legacy CI workflows
- Flatten v3/ contents to repo root
- Add Doppler secrets management (project=libnovel, config=prd)
- Add justfile with doppler run wrappers for all docker compose commands
- Strip hardcoded env fallbacks from docker-compose.yml
- Add minimal README.md
- Clean up .gitignore

ui/src/lib/utils.ts

@@ -0,0 +1,13 @@
+import { clsx, type ClassValue } from 'clsx';
+import { twMerge } from 'tailwind-merge';
+
+/**
+ * Merge Tailwind classes safely, resolving conflicts via tailwind-merge
+ * and collapsing falsy values via clsx.
+ *
+ * Usage:
+ *   cn('px-4 py-2', isActive && 'bg-brand', className)
+ */
+export function cn(...inputs: ClassValue[]): string {
+	return twMerge(clsx(inputs));
+}