feat: add dark/light theme toggle, fix cover display, refine overall style

- Add theme toggle button in navbar with sun/moon icons
- Define [data-theme=dark] CSS variables (warm dark palette)
- Persist theme preference in localStorage
- Follow system prefers-color-scheme on first visit
- Anti-flash inline script sets theme before render
- ECharts charts re-render with theme-aware colors on toggle
- Fix reading ranking covers: aspect-ratio 3/4 instead of fixed height
- Fix discover covers: aspect-ratio 3/4 for proper portrait display
- Replace purple hero gradient with warm brown/amber matching accent
- All hardcoded colors replaced with CSS variables
- Smooth 0.3s transitions on all theme-dependent elements
- Update footer text from SQLite to MySQL

templates/index.html

@@ -6,6 +6,17 @@
     <title>我的读书数据</title>
     <link rel="stylesheet" href="/static/css/style.css">
     <script src="https://cdn.jsdelivr.net/npm/echarts@5.5.0/dist/echarts.min.js"></script>
+    <!-- 防闪烁：在 CSS 加载后、页面渲染前设置主题 -->
+    <script>
+        (function() {
+            var saved = localStorage.getItem('theme');
+            if (saved) {
+                document.documentElement.setAttribute('data-theme', saved);
+            } else if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
+                document.documentElement.setAttribute('data-theme', 'dark');
+            }
+        })();
+    </script>
 </head>
 <body>
     <!-- 数据库连接告警横幅（MySQL 连接失败时显示） -->
@@ -30,7 +41,13 @@
                 <a href="#notes" class="nav-link">笔记</a>
                 <a href="#discover" class="nav-link">推荐</a>
             </div>
-            <div class="nav-update" id="fetchTime"></div>
+            <div class="nav-right">
+                <span class="nav-update" id="fetchTime"></span>
+                <button class="theme-toggle" onclick="toggleTheme()" title="切换明亮/黑暗模式" aria-label="切换主题">
+                    <span class="icon-sun">☀️</span>
+                    <span class="icon-moon">🌙</span>
+                </button>
+            </div>
         </div>
     </nav>
 
@@ -171,7 +188,7 @@
     <!-- 页脚 -->
     <footer class="footer">
         <div class="container">
-            <p>数据来源：微信读书 · 数据存储：本地 SQLite · 更新时间：<span id="footerTime"></span></p>
+            <p>数据来源：微信读书 · 数据存储：本地 MySQL · 更新时间：<span id="footerTime"></span></p>
             <p class="footer-hint">点击书籍封面可在微信读书 App 中打开</p>
         </div>
     </footer>