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/entrypoints/popup/App.tsx

@@ -0,0 +1,47 @@
+import RouterProvider from '@/providers/RouterProvider';
+import TopBar from '@/components/TopBar';
+import RouterContainer from '@/components/RouterContainer';
+import ErrorBoundary from '@/components/ErrorBoundary';
+import { SnackbarProvider } from '@/components/GlobalSnackbar';
+import { getEntryPointType } from '@/config/features';
+import { useMemo } from 'react';
+
+export default function App() {
+  const handleOpenOptions = () => {
+    chrome.runtime.openOptionsPage().catch(console.error);
+  };
+
+  const entryType = useMemo(() => getEntryPointType(), []);
+
+  const routerConfig = useMemo(() => {
+    if (entryType === 'tab') {
+      return {
+        syncKey: 'app/tabRoute' as const,
+        visiblePagesKey: 'app/tabVisiblePages' as const,
+        pageOrderKey: 'app/tabPageOrder' as const,
+      };
+    }
+    return {
+      syncKey: 'app/popupRoute' as const,
+      visiblePagesKey: 'app/popupVisiblePages' as const,
+      pageOrderKey: 'app/popupPageOrder' as const,
+    };
+  }, [entryType]);
+
+  return (
+    <RouterProvider
+      syncKey={routerConfig.syncKey}
+      visiblePagesKey={routerConfig.visiblePagesKey}
+      pageOrderKey={routerConfig.pageOrderKey}
+    >
+      <SnackbarProvider initialOptions={{ autoHideDuration: 1500 }}>
+        <div className="app flex flex-col w-[400px] max-w-[400px] min-w-[400px] h-[600px] min-h-[600px] overflow-hidden bg-background sm:w-screen sm:max-w-none sm:min-w-0 sm:h-screen sm:min-h-0">
+          <TopBar onOpenOptions={handleOpenOptions} />
+          <ErrorBoundary>
+            <RouterContainer />
+          </ErrorBoundary>
+        </div>
+      </SnackbarProvider>
+    </RouterProvider>
+  );
+}