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/pages/Dashboard/index.tsx

@@ -0,0 +1,41 @@
+import { useRouter } from '@/providers/RouterProvider';
+import ToolCard from '@/pages/Dashboard/ToolCard';
+import { getFeatureByKey } from '@/config/features';
+import type { PageType } from '@/types/storage';
+import { useMemo } from 'react';
+import { useI18n } from '@/utils/chromeI18n';
+import { cn } from '@/lib/utils';
+
+export default function DashboardPage() {
+  const { navigateTo, visiblePages, pageOrder } = useRouter();
+  const { t } = useI18n(['features']);
+
+  const visibleSet = useMemo(() => new Set<string>(visiblePages), [visiblePages]);
+
+  return (
+    <div
+      className={cn(
+        'grid grid-cols-1 sm:grid-cols-[repeat(auto-fill,minmax(290px,1fr))] auto-rows-auto gap-3.5 p-3.5 w-full h-auto',
+        'select-none',
+      )}
+    >
+      {pageOrder.map((key) => {
+        if (!visibleSet.has(key)) return null;
+
+        const feature = getFeatureByKey(key);
+        if (!feature?.themeColorKey || feature.icon == null) return null;
+
+        return (
+          <ToolCard
+            key={key}
+            title={t(feature.labelKey)}
+            description={t(feature.descriptionKey)}
+            colorKey={feature.themeColorKey}
+            icon={feature.icon}
+            onNavigate={() => navigateTo(key as PageType)}
+          />
+        );
+      })}
+    </div>
+  );
+}