feat: refine citations format and improve content presentation

Backend:
- Simplify citations prompt format and rules
- Add clear distinction between chat responses and file content
- Enforce full URL usage in markdown links, prohibit [cite-1] format
- Require content-first approach: write full content, then add citations at end

Frontend:
- Hide <citations> block in both chat messages and markdown preview
- Remove top-level Citations/Sources list for cleaner UI
- Auto-remove <citations> block in code editor view for markdown files
- Keep inline citation hover cards for reference details

This ensures citations are presented like Claude: clean content with inline reference badges.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/package.json

@@ -61,6 +61,7 @@
     "embla-carousel-react": "^8.6.0",
     "gsap": "^3.13.0",
     "hast": "^1.0.0",
+    "katex": "^0.16.28",
     "lucide-react": "^0.562.0",
     "motion": "^12.26.2",
     "nanoid": "^5.1.6",
@@ -71,6 +72,8 @@
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
     "react-resizable-panels": "^4.4.1",
+    "rehype-katex": "^7.0.1",
+    "remark-math": "^6.0.0",
     "shiki": "3.15.0",
     "sonner": "^2.0.7",
     "streamdown": "1.4.0",