chore: move src/llama_stack/ui to src/llama_stack_ui (#4068)

# What does this PR do?
This better separates UI from backend code, which was a point of
confusion often for our beloved AI friends.


## Test Plan
CI

src/llama_stack_ui/components/chat-playground/interrupt-prompt.tsx

@@ -0,0 +1,41 @@
+"use client";
+
+import { AnimatePresence, motion } from "framer-motion";
+import { X } from "lucide-react";
+
+interface InterruptPromptProps {
+  isOpen: boolean;
+  close: () => void;
+}
+
+export function InterruptPrompt({ isOpen, close }: InterruptPromptProps) {
+  return (
+    <AnimatePresence>
+      {isOpen && (
+        <motion.div
+          initial={{ top: 0, filter: "blur(5px)" }}
+          animate={{
+            top: -40,
+            filter: "blur(0px)",
+            transition: {
+              type: "spring",
+              filter: { type: "tween" },
+            },
+          }}
+          exit={{ top: 0, filter: "blur(5px)" }}
+          className="absolute left-1/2 flex -translate-x-1/2 overflow-hidden whitespace-nowrap rounded-full border bg-background py-1 text-center text-sm text-muted-foreground"
+        >
+          <span className="ml-2.5">Press Enter again to interrupt</span>
+          <button
+            className="ml-1 mr-2.5 flex items-center"
+            type="button"
+            onClick={close}
+            aria-label="Close"
+          >
+            <X className="h-3 w-3" />
+          </button>
+        </motion.div>
+      )}
+    </AnimatePresence>
+  );
+}