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>

wxt.config.ts

@@ -44,6 +44,7 @@ function manualChunksForHtmlOnly(): Plugin {
 
 export default defineConfig({
   modules: ['@wxt-dev/module-react'],
+  srcDir: 'src',
   manifest: {
     name: 'Testing Tool',
     description: 'A tool for testing web applications.',