i18n: 中文化所有错误消息和用户交互文本

- Go 后端 handler 错误消息统一中文化（middleware/repos/generate/review/settings）
- 前端 JS 加载和错误消息中文化（diff-viewer.js, graph.js）
- 模板页面错误 fallback 和静态文本中文化（index/generate/review/base）
- 消除中英文混杂，提升中文用户体验

templates/layouts/base.html

@@ -76,7 +76,7 @@
 {{define "footer"}}
 <footer class="bg-white border-t border-gray-200 py-4">
     <div class="max-w-7xl mx-auto px-4 sm:px-6 lg:px-8 text-center text-sm text-gray-400">
-        PR-Helper — AI-powered PR description & code review
+        PR-Helper — AI 驱动的 PR 描述与代码审查
     </div>
 </footer>
 {{end}}

templates/pages/generate.html

@@ -92,7 +92,7 @@
         async function loadRefs() {
             try {
                 const resp = await fetch(`/api/repos/${repoId}/refs`, { credentials: 'same-origin' });
-                if (!resp.ok) throw new Error('Failed to load refs');
+                if (!resp.ok) throw new Error('加载分支列表失败');
                 const refs = await resp.json();
 
                 // Fetch recent commits for the "Recent Commits" group

templates/pages/index.html

@@ -142,7 +142,7 @@ document.getElementById('clone-form').addEventListener('submit', async function(
 
         if (!resp.ok) {
             const data = await resp.json();
-            throw new Error(data.error || 'clone failed');
+            throw new Error(data.error || '克隆失败');
         }
 
         const reader = resp.body.getReader();

templates/pages/review.html

@@ -223,7 +223,7 @@
         async function loadRefs() {
             try {
                 const resp = await fetch(`/api/repos/${repoId}/refs`, { credentials: 'same-origin' });
-                if (!resp.ok) throw new Error('Failed to load refs');
+                if (!resp.ok) throw new Error('加载分支列表失败');
                 const refs = await resp.json();
 
                 // Fetch recent commits for the "Recent Commits" group
@@ -576,7 +576,7 @@
             try {
                 // Fetch the review result
                 const resp = await fetch(`/api/repos/${repoId}/review/analyses/${analysisId}`, { credentials: 'same-origin' });
-                if (!resp.ok) throw new Error('Failed to load review');
+                if (!resp.ok) throw new Error('加载评审数据失败');
                 const data = await resp.json();
 
                 const result = data.result;