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/jest.setup.ts

@@ -0,0 +1,23 @@
+// Import llama-stack-client shims for Node environment
+import "llama-stack-client/shims/node";
+
+// Add any other global test setup here
+import "@testing-library/jest-dom";
+
+// Mock ResizeObserver globally
+global.ResizeObserver = class ResizeObserver {
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+};
+
+// Mock IntersectionObserver globally
+global.IntersectionObserver = class IntersectionObserver {
+  constructor(callback: IntersectionObserverCallback) {}
+  observe() {}
+  unobserve() {}
+  disconnect() {}
+  takeRecords() {
+    return [];
+  }
+} as any;