fix: Ollama model validation, TUI CSS, WebUI overhaul

1. Ollama: validate model is configured before API call, show helpful error
2. TUI: remove unsupported scroll-behavior CSS property
3. WebUI server: add full tool calling loop (FC + JSON action)
4. WebUI HTML: complete redesign
   - Modern dark theme with better typography
   - Markdown rendering via marked.js
   - Tool call/result visualization with colored cards
   - Dynamic sidebar (engines + tools from server)
   - Better input area with gradient send button
   - Responsive layout

xcli_core/tui.py

@@ -62,8 +62,7 @@ if TEXTUAL_AVAILABLE:
         scrollbar-color: {_Theme.BORDER};
         scrollbar-color-hover: {_Theme.TEXT_MUTED};
         padding: 0 1;
-        /* 平滑滚动 */
-        scroll-behavior: smooth;
+
     }}
 
     /* ── 输入区域 ── */