feat: 对话列表显示总数，SOUL 编辑区高度自适应收敛

- 新增 POST /threads/count：与 /threads/search 同一套过滤（标题搜索、
  系统会话排除），仅返回 {total}；ThreadMetaStore 增加 count 方法
  （SQL 走 COUNT(*)，JSON 过滤分批扫描计数）
- 对话页面搜索框下常驻显示「共 N 条对话」，分页条显示 当前页/总页数
  （useThreadsCount，仅随筛选条件变化，翻页不重复请求）
- 智能体右侧 SOUL 预览改为 40vh 上限自适应展开（不再全部展开）；
  详情面板/卡片编辑/创建页的 SOUL 输入框统一为适中高度 + 可拖拽

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend-web/src/components/workspace/agents/agent-card.tsx

@@ -921,7 +921,7 @@ export function AgentCard({ agent, layout = "card", onPublisherClick }: AgentCar
                     ? "正在加载 SOUL.md..."
                     : "输入 SOUL.md 内容"
                 }
-                className="min-h-[40vh] resize-y font-mono text-xs"
+                className="max-h-[45vh] min-h-48 resize-y font-mono text-xs"
               />
             </div>
             <div className="space-y-2">

frontend-web/src/components/workspace/agents/agent-detail-panel.tsx

@@ -577,13 +577,13 @@ export function AgentDetailPanel({
                 setDraft({ ...draft, soul: e.currentTarget.value })
               }
               placeholder="输入 SOUL.md 内容"
-              className="min-h-[45vh] resize-y font-mono text-xs"
+              className="max-h-[45vh] min-h-48 resize-y font-mono text-xs"
             />
           ) : (
-            // No inner max-height: the SOUL text flows at full length and the
-            // panel body scrolls as a whole, instead of a small nested scroll
-            // box on an otherwise empty screen.
-            <p className="text-muted-foreground whitespace-pre-wrap text-sm leading-relaxed">
+            // Viewport-relative cap: the SOUL preview expands with the screen
+            // (taller than the old fixed 240px box) but never swallows the
+            // whole panel — long content still scrolls inside.
+            <p className="text-muted-foreground max-h-[40vh] overflow-y-auto whitespace-pre-wrap text-sm leading-relaxed">
               {soulPreview}
             </p>
           )}