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/cli/stack/run.py

@@ -253,7 +253,7 @@ class StackRun(Subcommand):
             )
             return
 
-        ui_dir = REPO_ROOT / "llama_stack" / "ui"
+        ui_dir = REPO_ROOT / "llama_stack_ui"
         logs_dir = Path("~/.llama/ui/logs").expanduser()
         try:
             # Create logs directory if it doesn't exist