rebrand to orc-gallery and optimise search/thumbnail responsiveness

Rebrand:
- Rename project, binary, desktop file, and icons from screenshot-gallery/orcs-gallery to orc-gallery
- Update display name to "ORC Gallery" throughout (window title, .desktop, README, plan.md)
- Config dir changed from ScreenshotOCRGallery to OrcGallery

Performance:
- cancelSearch() is now non-blocking; new searches start immediately instead of waiting for the previous thread to drain
- Remove double-emit from QFutureWatcher::finished handler; results emitted once directly from background thread
- SQLite WAL mode + NORMAL sync + 20 MB page cache + memory temp store + 256 MB mmap applied on init and per-thread connections
- Typing inactivity timer reduced from 500 ms to 150 ms
- COUNT(*) OVER() window function folds total-count into the main SELECT, eliminating a separate COUNT round-trip per search
- ImageThumbnail::paintEvent draws the filename overlay; removes 3 child QObjects (QFrame + QLabel + QHBoxLayout) per thumbnail
- loadThumbnailAsync posts QImage load+scale to QThreadPool; QPixmap::fromImage called on main thread via Qt::QueuedConnection invokeMethod; QPointer guards against use-after-free on gallery clear

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

build.sh

@@ -80,8 +80,8 @@ run_application() {
   print_message "Running application..."
 
   # Check if the application exists
-  if [ -f "build/screenshot-gallery" ]; then
-    ./build/screenshot-gallery
+  if [ -f "build/orc-gallery" ]; then
+    ./build/orc-gallery
   else
     print_error "Application not found. Build might have failed."
     exit 1
@@ -89,7 +89,7 @@ run_application() {
 }
 
 # -------- Main script execution --------
-print_message "Starting build process for Screenshot Gallery..."
+print_message "Starting build process for ORC Gallery..."
 
 check_dependencies
 create_build_dir
@@ -104,5 +104,5 @@ echo
 if [[ $REPLY =~ ^[Yy]$ ]]; then
   run_application
 else
-  print_message "You can run the application later using: ./build/screenshot-gallery"
+  print_message "You can run the application later using: ./build/orc-gallery"
 fi