refactor: reorganize directory structure into src/

Move all source code directories into src/ for cleaner project structure:
- pages/, components/, utils/, config/, providers/, types/, lib/, assets/, entrypoints/ → src/
- Use WXT srcDir config to resolve @/ alias to src/
- Update tsconfig, vitest, eslint, tailwind configs
- Remove scattered README.md files from subdirectories
- Update documentation (AGENTS.md, CODING_STANDARDS.md, README.md)
- Fix pre-existing lint error in RouterProvider.tsx

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/providers/AppRoot.tsx

@@ -0,0 +1,17 @@
+import React from 'react';
+import { ThemeModeProvider } from './ThemeModeProvider';
+import { RouterProvider } from './RouterProvider'; // 💡 1. 全局合龙：引入满血重构的路由中枢
+
+interface AppRootProps {
+  children: React.ReactNode;
+}
+
+export default function AppRoot({ children }: AppRootProps) {
+  return (
+    <React.StrictMode>
+      <ThemeModeProvider>
+        <RouterProvider>{children}</RouterProvider>
+      </ThemeModeProvider>
+    </React.StrictMode>
+  );
+}