fix: BUG-012/013/007 — panel overlap, Markdown rendering, authStore tests

BUG-012: Reposition side panel toggle button (top-[52px]→top-20) to
avoid overlap with header buttons in ResizableChatLayout.

BUG-013: Install @tailwindcss/typography plugin and import in index.css
to enable prose-* Markdown rendering classes in StreamingText.

BUG-007: Rewrite authStore tests to match HttpOnly cookie auth model
(login takes 1 arg, no token/refreshToken in state). Rewrite request
interceptor tests for cookie-based auth. Update bug-tracker status.

desktop/src/components/ai/ResizableChatLayout.tsx

@@ -66,7 +66,7 @@ export function ResizableChatLayout({
         {chatPanel}
         <button
           onClick={handleToggle}
-          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"
+          className="absolute top-20 right-4 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-[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"
+              className="absolute top-20 right-4 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" />