fix: adapt frontend image paths for backend port 8002 and use blob download

Backend port changed from 8000 to 8002 due to port conflict (frpc on Windows).
- Add resolveImageUrl() to convert relative image URLs to full backend URLs
- Fix all spatial page image src and download handlers to use backend address
- Fix chat message image rendering to use correct port
- Switch download to blob-based approach to support cross-origin file saving

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/pyproject.toml

@@ -43,6 +43,7 @@ dependencies = [
     "docx2txt>=0.9",
     "pypdf>=6.12.0",
     "pymupdf>=1.27.2.3",
+    "pdf2image>=1.17.0",
     "opencv-python-headless>=4.13.0.92",
 ]