chore: optimize SEO and fix golangci-lint issues

- Implement dynamic SEO meta tags and sitemap.xml for toolbox.pengzhan.dev
- Fix SPA title/meta refresh issue in layout.html
- Address golangci-lint findings:
  - Refactor switch-case for tool path and SVG token parsing
  - Replace math.Pow with direct multiplication for performance
  - Fix unhandled error returns and unused imports
  - Omit redundant type declarations

cmd/toolbox/web/layout.html

@@ -4,6 +4,24 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>{{.Title}} | Own-Tools</title>
+    <meta name="description" content="{{.Description}}">
+    <meta name="keywords" content="{{.Keywords}}">
+    <link rel="canonical" href="{{.CanonicalURL}}">
+
+    <!-- Open Graph / Facebook -->
+    <meta property="og:type" content="website">
+    <meta property="og:url" content="{{.CanonicalURL}}">
+    <meta property="og:title" content="{{.Title}} | Own-Tools">
+    <meta property="og:description" content="{{.Description}}">
+    <meta property="og:image" content="/static/og-image.png">
+
+    <!-- Twitter -->
+    <meta property="twitter:card" content="summary_large_image">
+    <meta property="twitter:url" content="{{.CanonicalURL}}">
+    <meta property="twitter:title" content="{{.Title}} | Own-Tools">
+    <meta property="twitter:description" content="{{.Description}}">
+    <meta property="twitter:image" content="/static/og-image.png">
+
     <link rel="icon" type="image/svg+xml" href="data:image/svg+xml,<svg xmlns=%22http://www.w3.org/2000/svg%22 viewBox=%220 0 100 100%22><text y=%22.9em%22 font-size=%2290%22>🖋️</text></svg>">
     
     {{if .GA_ID}}
@@ -126,10 +144,41 @@
             renderCurrentPath();
         }
 
+        const metaConfig = {
+            'welcome': {
+                title: '探索工具箱',
+                desc: '基于 Go 语言构建的模块化个人工具箱，提供汉字字帖生成、数字学习等多种实用生产力工具。',
+                keywords: '个人工具箱, 汉字字帖生成, 书法练习, 数字学习, Own-Tools'
+            },
+            'zitie': {
+                title: '汉字字帖生成器 - 教学方格与步进式分解',
+                desc: '在线生成 2x3 教学方格字帖和 9 列步进式笔顺分解字帖，支持多种书法字体和古风排版。',
+                keywords: '字帖生成, 笔顺分解, 汉字教学, 书法字帖, 练字'
+            },
+            'learn-number': {
+                title: '趣味数字学习工具',
+                desc: '为儿童设计的数字学习与计数练习工具，生动活泼，寓教于乐。',
+                keywords: '数字学习, 儿童计数, 幼小衔接'
+            }
+        };
+
         function renderCurrentPath() {
             const path = window.location.pathname.replace('/', '') || 'welcome';
             
-            // 隐藏所有面板
+            // 1. 更新标题与元数据
+            const meta = metaConfig[path] || metaConfig['welcome'];
+            document.title = `${meta.title} | Own-Tools`;
+            document.querySelector('meta[name="description"]').setAttribute('content', meta.desc);
+            document.querySelector('meta[name="keywords"]').setAttribute('content', meta.keywords);
+            
+            // 更新 Open Graph 标签 (可选，社交分享主要看服务端渲染)
+            const canonical = `https://toolbox.pengzhan.dev/${path === 'welcome' ? '' : path}`;
+            document.querySelector('link[rel="canonical"]').setAttribute('href', canonical);
+            document.querySelector('meta[property="og:title"]').setAttribute('content', `${meta.title} | Own-Tools`);
+            document.querySelector('meta[property="og:description"]').setAttribute('content', meta.desc);
+            document.querySelector('meta[property="og:url"]').setAttribute('content', canonical);
+
+            // 2. 隐藏所有面板
             document.querySelectorAll('.tool-panel').forEach(p => p.classList.remove('active'));
             
             // 显示目标面板