fix(.gitignore): anchor Python build artifacts; recover panel/src/lib (28 files)

The 'lib/' rule (intended for Python virtualenv at repo root) was matching panel/src/lib/, hiding the entire panel API client + utility tree from git. Anchored Python build-artifact rules to the repo root with a leading slash so they only match at the top level. Adds 28 panel/src/lib files that should have been tracked from day one.

panel/src/lib/utils.ts

@@ -0,0 +1,6 @@
+import { clsx, type ClassValue } from "clsx"
+import { twMerge } from "tailwind-merge"
+
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs))
+}