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/utils/__tests__/textStatistics.test.ts

@@ -0,0 +1,60 @@
+import { describe, expect, it } from 'vitest';
+import { formatByteSize, getTextStats } from '@/utils/textStatistics';
+
+describe('textStatistics utils', () => {
+  describe('getTextStats', () => {
+    it('should return zeros for empty text', () => {
+      const stats = getTextStats('');
+      expect(stats).toEqual({ characters: 0, words: 0, lines: 0, bytes: 0 });
+    });
+
+    it('should count characters correctly', () => {
+      expect(getTextStats('abc').characters).toBe(3);
+      expect(getTextStats('a b c').characters).toBe(5);
+      expect(getTextStats('你好').characters).toBe(2);
+    });
+
+    it('should count English words correctly', () => {
+      expect(getTextStats('hello world').words).toBe(2);
+      expect(getTextStats('  hello   world  ').words).toBe(2);
+      expect(getTextStats('hello, world!').words).toBe(2);
+    });
+
+    it('should count Chinese words correctly', () => {
+      // "你好世界" 在 Intl.Segmenter 中通常被识别为 "你好" 和 "世界" 两个词
+      const stats = getTextStats('你好世界');
+      expect(stats.words).toBe(2);
+    });
+
+    it('should count mixed language words correctly', () => {
+      const stats = getTextStats('Hello 你好');
+      // "Hello" (1) + "你好" (1) = 2
+      expect(stats.words).toBe(2);
+    });
+
+    it('should count lines correctly', () => {
+      expect(getTextStats('line1\nline2').lines).toBe(2);
+      expect(getTextStats('line1\nline2\n').lines).toBe(3);
+    });
+
+    it('should count bytes correctly (UTF-8)', () => {
+      expect(getTextStats('abc').bytes).toBe(3);
+      expect(getTextStats('你好').bytes).toBe(6); // UTF-8 中每个常用汉字占 3 字节
+    });
+
+    it('should handle special cases', () => {
+      expect(getTextStats('   ').words).toBe(0);
+      expect(getTextStats('\n\n\n').lines).toBe(4);
+      expect(getTextStats('\n\n\n').words).toBe(0);
+    });
+  });
+
+  describe('formatByteSize', () => {
+    it('should format bytes correctly', () => {
+      expect(formatByteSize(100)).toBe('100 B');
+      expect(formatByteSize(0)).toBe('0 B');
+      expect(formatByteSize(1024)).toBe('1.0 KB');
+      expect(formatByteSize(1024 * 1024)).toBe('1.00 MB');
+    });
+  });
+});