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>

OCR-scripts/ocr_screenshots.py

@@ -7,8 +7,8 @@ Usage:
 
 Arguments:
     --db              Path to the SQLite database file.
-                      Default: $XDG_DATA_HOME/screenshot-gallery/screenshot_ocr.db
-                               (~/.local/share/screenshot-gallery/screenshot_ocr.db)
+                      Default: $XDG_DATA_HOME/orc-gallery/screenshot_ocr.db
+                               (~/.local/share/orc-gallery/screenshot_ocr.db)
     --screenshots-dir Directory containing screenshot images to process.
                       Default: ~/Screenshots
 """
@@ -53,7 +53,7 @@ signal.signal(signal.SIGINT,  _handle_signal)
 # ---------------------------------------------------------------------------
 def parse_args() -> argparse.Namespace:
     xdg_data = os.environ.get("XDG_DATA_HOME", os.path.expanduser("~/.local/share"))
-    default_db  = os.path.join(xdg_data, "screenshot-gallery", "screenshot_ocr.db")
+    default_db  = os.path.join(xdg_data, "orc-gallery", "screenshot_ocr.db")
     default_dir = os.path.expanduser("~/Screenshots")
 
     parser = argparse.ArgumentParser(