chore(package): migrate to src/ layout (#3920)

Migrates package structure to src/ layout following Python packaging
best practices.

All code moved from `llama_stack/` to `src/llama_stack/`. Public API
unchanged - imports remain `import llama_stack.*`.

Updated build configs, pre-commit hooks, scripts, and GitHub workflows
accordingly. All hooks pass, package builds cleanly.

**Developer note**: Reinstall after pulling: `pip install -e .`

src/llama_stack/ui/hooks/use-mobile.ts

@@ -0,0 +1,21 @@
+import * as React from "react";
+
+const MOBILE_BREAKPOINT = 768;
+
+export function useIsMobile() {
+  const [isMobile, setIsMobile] = React.useState<boolean | undefined>(
+    undefined
+  );
+
+  React.useEffect(() => {
+    const mql = window.matchMedia(`(max-width: ${MOBILE_BREAKPOINT - 1}px)`);
+    const onChange = () => {
+      setIsMobile(window.innerWidth < MOBILE_BREAKPOINT);
+    };
+    mql.addEventListener("change", onChange);
+    setIsMobile(window.innerWidth < MOBILE_BREAKPOINT);
+    return () => mql.removeEventListener("change", onChange);
+  }, []);
+
+  return !!isMobile;
+}