feat(ui): implement dark/light theme switch

reaction_board/web/static/index.html

@@ -9,6 +9,7 @@
   <title>咻咻满 Reaction 看板</title>
   <link rel="icon" href="/static/favicon.svg" type="image/svg+xml">
   <link rel="stylesheet" href="/static/style.css">
+  <script src="/static/theme.js"></script>
 </head>
 <body>
   <div class="app" id="app">
@@ -23,6 +24,7 @@
       <div class="topbar-actions">
         <a class="topbar-hint topbar-auth" href="/login" hidden>登录</a>
         <span class="topbar-links" data-topbar-links hidden></span>
+        <button class="theme-toggle" type="button" data-theme-toggle aria-label="切换主题"></button>
       </div>
     </header>
     <div class="preview-banner" id="preview-banner" hidden>预览数据 · 不影响真实数据库</div>
@@ -41,6 +43,7 @@
         <div class="topbar-actions">
           <a class="topbar-hint topbar-auth" href="/login" hidden>登录</a>
           <span class="topbar-links" data-topbar-links hidden></span>
+          <button class="theme-toggle" type="button" data-theme-toggle aria-label="切换主题"></button>
         </div>
       </div>
       <div class="nav-items" id="nav-items"></div>