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>

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