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>

tailwind.config.js

@@ -1,14 +1,7 @@
 /** @type {import('tailwindcss').Config} */
 export default {
   darkMode: 'class',
-  content: [
-    './entrypoints/**/*.{js,ts,jsx,tsx}',
-    './components/**/*.{js,ts,jsx,tsx}',
-    './pages/**/*.{js,ts,jsx,tsx}',
-    './providers/**/*.{js,ts,jsx,tsx}',
-    './utils/**/*.{js,ts,jsx,tsx}',
-    './config/**/*.{js,ts,jsx,tsx}',
-  ],
+  content: ['./entrypoints/**/*.{js,ts,jsx,tsx}', './src/**/*.{js,ts,jsx,tsx}'],
   theme: {
     extend: {
       colors: {