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/uv.lock

@@ -1,5 +1,5 @@
 version = 1
-revision = 2
+revision = 3
 requires-python = ">=3.12"
 resolution-markers = [
     "python_full_version >= '3.13'",
@@ -441,6 +441,7 @@ dependencies = [
     { name = "opencv-python-headless" },
     { name = "pandas" },
     { name = "passlib", extra = ["bcrypt"] },
+    { name = "pdf2image" },
     { name = "psycopg", extra = ["binary"] },
     { name = "pydantic", extra = ["email"] },
     { name = "pydantic-settings" },
@@ -492,6 +493,7 @@ requires-dist = [
     { name = "opencv-python-headless", specifier = ">=4.13.0.92" },
     { name = "pandas", specifier = ">=2.2.3" },
     { name = "passlib", extras = ["bcrypt"], specifier = ">=1.7.4" },
+    { name = "pdf2image", specifier = ">=1.17.0" },
     { name = "psycopg", extras = ["binary"], specifier = ">=3.1.0" },
     { name = "pydantic", extras = ["email"], specifier = ">=2.5.0" },
     { name = "pydantic-settings", specifier = ">=2.1.0" },
@@ -2396,6 +2398,18 @@ wheels = [
     { url = "https://mirrors.aliyun.com/pypi/packages/cc/20/ff623b09d963f88bfde16306a54e12ee5ea43e9b597108672ff3a408aad6/pathspec-0.12.1-py3-none-any.whl", hash = "sha256:a0d503e138a4c123b27490a4f7beda6a01c6f288df0e4a8b79c7eb0dc7b4cc08" },
 ]
 
+[[package]]
+name = "pdf2image"
+version = "1.17.0"
+source = { registry = "https://mirrors.aliyun.com/pypi/simple/" }
+dependencies = [
+    { name = "pillow" },
+]
+sdist = { url = "https://mirrors.aliyun.com/pypi/packages/00/d8/b280f01045555dc257b8153c00dee3bc75830f91a744cd5f84ef3a0a64b1/pdf2image-1.17.0.tar.gz", hash = "sha256:eaa959bc116b420dd7ec415fcae49b98100dda3dd18cd2fdfa86d09f112f6d57" }
+wheels = [
+    { url = "https://mirrors.aliyun.com/pypi/packages/62/33/61766ae033518957f877ab246f87ca30a85b778ebaad65b7f74fa7e52988/pdf2image-1.17.0-py3-none-any.whl", hash = "sha256:ecdd58d7afb810dffe21ef2b1bbc057ef434dabbac6c33778a38a3f7744a27e2" },
+]
+
 [[package]]
 name = "pillow"
 version = "11.3.0"