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

static/js/diff-viewer.js

@@ -298,7 +298,7 @@ const DiffViewer = {
         this._onRenderComplete = options.onComplete || null;
 
         if (!files || files.length === 0) {
-            this.container.innerHTML = '<div class="text-gray-500 p-4">No changes</div>';
+            this.container.innerHTML = '<div class="text-gray-500 p-4">暂无变更</div>';
             return;
         }
 
@@ -664,12 +664,12 @@ const DiffViewer = {
     async loadDiff(repoId, base, head, options = {}) {
         if (!this.container) return;
 
-        this.container.innerHTML = '<div class="text-center p-8"><div class="spinner inline-block"></div><p class="mt-2 text-gray-500">Loading diff...</p></div>';
+        this.container.innerHTML = '<div class="text-center p-8"><div class="spinner inline-block"></div><p class="mt-2 text-gray-500">加载 Diff 中...</p></div>';
 
         try {
             const url = `/api/repos/${repoId}/diff?base=${encodeURIComponent(base)}&head=${encodeURIComponent(head)}`;
             const resp = await fetch(url, { credentials: 'same-origin' });
-            if (!resp.ok) throw new Error('Failed to load diff');
+            if (!resp.ok) throw new Error('加载 Diff 失败');
 
             const data = await resp.json();
 
@@ -679,24 +679,24 @@ const DiffViewer = {
                 this.renderFiles(data, options);
             }
         } catch (err) {
-            this.container.innerHTML = `<div class="text-red-500 p-4">Error loading diff: ${err.message}</div>`;
+            this.container.innerHTML = `<div class="text-red-500 p-4">加载 Diff 失败: ${err.message}</div>`;
         }
     },
 
     async loadFiles(repoId, base, head) {
         if (!this.container) return;
 
-        this.container.innerHTML = '<div class="text-center p-8"><div class="spinner inline-block"></div><p class="text-gray-500">Loading files...</p></div>';
+        this.container.innerHTML = '<div class="text-center p-8"><div class="spinner inline-block"></div><p class="text-gray-500">加载文件列表中...</p></div>';
 
         try {
             const url = `/api/repos/${repoId}/diff?base=${encodeURIComponent(base)}&head=${encodeURIComponent(head)}&per_file=true`;
             const resp = await fetch(url, { credentials: 'same-origin' });
-            if (!resp.ok) throw new Error('Failed to load diff');
+            if (!resp.ok) throw new Error('加载 Diff 失败');
 
             const files = await resp.json();
             this.renderFiles(files);
         } catch (err) {
-            this.container.innerHTML = `<div class="text-red-500 p-4">Error: ${err.message}</div>`;
+            this.container.innerHTML = `<div class="text-red-500 p-4">加载失败: ${err.message}</div>`;
         }
     },