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/playwright.config.ts

@@ -0,0 +1,36 @@
+import { defineConfig, devices } from "@playwright/test";
+
+/**
+ * See https://playwright.dev/docs/test-configuration.
+ */
+export default defineConfig({
+  testDir: "./e2e",
+  /* Run tests in files in parallel */
+  fullyParallel: true,
+  /* Reporter to use. See https://playwright.dev/docs/test-reporters */
+  reporter: "line",
+  /* Shared settings for all the projects below. See https://playwright.dev/docs/api/class-testoptions. */
+  use: {
+    /* Base URL to use in actions like `await page.goto('/')`. */
+    baseURL: "http://localhost:8322",
+
+    /* Collect trace when retrying the failed test. See https://playwright.dev/docs/trace-viewer */
+    trace: "on-first-retry",
+  },
+
+  /* Configure projects for major browsers */
+  projects: [
+    {
+      name: "chromium",
+      use: { ...devices["Desktop Chrome"] },
+    },
+  ],
+
+  /* Run your local dev server before starting the tests */
+  webServer: {
+    command: "npm run dev",
+    url: "http://localhost:8322",
+    reuseExistingServer: !process.env.CI,
+    timeout: 120 * 1000,
+  },
+});