fix(ui): button overlap + Markdown rendering (BUG-012, BUG-013)

BUG-012: Move side panel toggle button below header (top-3 → top-[52px])
to avoid overlap with "详情" button in chat header.

BUG-013: Add rich Markdown component overrides to StreamingText:
- Code blocks: dark bg, border, rounded, overflow-x-auto
- Inline code: subtle bg highlight
- Tables: full borders, alternating header bg, proper padding
- Lists: disc/decimal markers, spacing
- Headings: proper hierarchy sizes
- Blockquotes: left border + subtle bg
- Links: blue underlined with hover

desktop/src/components/ai/ResizableChatLayout.tsx

@@ -66,7 +66,7 @@ export function ResizableChatLayout({
         {chatPanel}
         <button
           onClick={handleToggle}
-          className="absolute top-3 right-3 z-10 p-1.5 rounded-md bg-white/80 dark:bg-gray-800/80 border border-gray-200 dark:border-gray-700 text-gray-500 dark:text-gray-400 hover:text-gray-700 dark:hover:text-gray-200 hover:bg-white dark:hover:bg-gray-800 transition-colors shadow-sm"
+          className="absolute top-[52px] right-3 z-10 p-1.5 rounded-md bg-white/80 dark:bg-gray-800/80 border border-gray-200 dark:border-gray-700 text-gray-500 dark:text-gray-400 hover:text-gray-700 dark:hover:text-gray-200 hover:bg-white dark:hover:bg-gray-800 transition-colors shadow-sm"
           title="打开侧面板"
         >
           <PanelRightOpen className="w-4 h-4" />
@@ -91,7 +91,7 @@ export function ResizableChatLayout({
             {chatPanel}
             <button
               onClick={handleToggle}
-              className="absolute top-3 right-3 z-10 p-1.5 rounded-md bg-white/80 dark:bg-gray-800/80 border border-gray-200 dark:border-gray-700 text-gray-500 dark:text-gray-400 hover:text-gray-700 dark:hover:text-gray-200 hover:bg-white dark:hover:bg-gray-800 transition-colors shadow-sm"
+              className="absolute top-[52px] right-3 z-10 p-1.5 rounded-md bg-white/80 dark:bg-gray-800/80 border border-gray-200 dark:border-gray-700 text-gray-500 dark:text-gray-400 hover:text-gray-700 dark:hover:text-gray-200 hover:bg-white dark:hover:bg-gray-800 transition-colors shadow-sm"
               title="关闭侧面板"
             >
               <PanelRightClose className="w-4 h-4" />