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/index.css

@@ -1,4 +1,5 @@
 @import "tailwindcss";
+@import "@tailwindcss/typography";
 
 :root {
   /* Brand Colors - 中性灰色系 */