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/package.json

@@ -63,6 +63,7 @@
   "devDependencies": {
     "@eslint/js": "^10.0.1",
     "@playwright/test": "^1.58.2",
+    "@tailwindcss/typography": "^0.5.19",
     "@tailwindcss/vite": "^4.2.2",
     "@tauri-apps/cli": "^2.10.1",
     "@testing-library/jest-dom": "6.6.3",