feat: UI全局美化重设计 + 恢复CI/测试/文档

UI 改进:
- 设置页面内嵌显示（不再弹窗）
- 临时邮箱卡片添加验证码提取按钮
- 导入账号弹窗 textarea 样式优化
- 账号头像 8 色渐变轮转
- 临时邮箱页顶栏邮箱点击复制
- 邮箱管理页自动选中默认分组
- 切换分组时重置邮件列 UI
- 修复收件箱/垃圾邮件 Tab 切换（.folder-tab → .email-tab）
- 修复从临时邮箱返回后账号列数据残留
- 仪表盘分组列表可点击导航

恢复:
- 恢复 .github/ CI 工作流
- 恢复 tests/ 测试目录
- 恢复 docs/ 文档目录
- 恢复 CONTRIBUTING.md, SECURITY.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/python-tests.yml

@@ -0,0 +1,48 @@
+name: Python Tests
+
+on:
+  push:
+    branches: [ "main", "master" ]
+    paths:
+      - "**.py"
+      - "requirements.txt"
+      - "tests/**"
+      - ".github/workflows/python-tests.yml"
+  pull_request:
+    paths:
+      - "**.py"
+      - "requirements.txt"
+      - "tests/**"
+      - ".github/workflows/python-tests.yml"
+  workflow_dispatch:
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: [ "3.11", "3.12", "3.13" ]
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install -r requirements.txt
+
+      - name: Syntax check
+        run: |
+          python -m py_compile web_outlook_app.py
+          python -m py_compile outlook_mail_reader.py
+
+      - name: Run unit tests
+        run: python -m unittest discover -s tests -v
+