Add bulk multi-photo upload to the desktop app

Mirror the Android bulk feature in the Qt uploader: choose several photos
at once, each a card with its own caption, date, and upload status, and
post the batch.

- Per-photo cards (PhotoItemWidget) in a scrollable list; retry re-posts
  only the failures.
- One upload per photo with a per-item UUID slug suffix so a batch posted
  in the same second cannot collide.
- Image prep runs off the UI thread via QtConcurrent so the window stays
  responsive.
- A "Simultaneous uploads" setting (1–8, default 3) caps how many photos
  are prepared/uploaded at once; the rest queue and fill freed slots.

desktop/CMakeLists.txt

@@ -5,12 +5,13 @@ set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_AUTOMOC ON)
 
-find_package(Qt6 REQUIRED COMPONENTS Widgets Network)
+find_package(Qt6 REQUIRED COMPONENTS Widgets Network Concurrent)
 find_package(Qt6Keychain REQUIRED)
 
 add_executable(uploader
     main.cpp
     mainwindow.cpp
+    photoitemwidget.cpp
     settings.cpp
     uploadclient.cpp
     imageprep.cpp
@@ -19,5 +20,6 @@ add_executable(uploader
 target_link_libraries(uploader PRIVATE
     Qt6::Widgets
     Qt6::Network
+    Qt6::Concurrent
     Qt6Keychain::Qt6Keychain
 )