feat: add macOS (darwin) platform support

[jianghuawang]

Summary

• Full macOS platform layer (darwin_api.py, key_translator.py) implementing the existing Api interface — shortcuts, startfile, hardware control, accessibility/screen recording permission checks
• PySide2→PySide6 compatibility shim so the existing PySide2/Qt5 codebase runs on PySide6/Qt6 without code changes
• Qt5Compat QML overlay for QtQuick.Dialogs version differences
• Cross-platform entry point (main.py) — MessageBox, stdout/stderr, Qt plugin paths now handle all three platforms
• On-demand OCR engine installer with first-launch download dialog
• darwin_RapidOCR-json OCR plugin using RapidOCR with CoreML execution provider
• macOS build pipeline (build.sh, create_dmg.sh) producing .app bundle and DMG

Bug fixes (not macOS-specific)

• Fix PDF text layer missing on PyMuPDF 1.27+ — fontsize became keyword-only in page.insert_text(); the old positional call silently failed, producing PDFs with no selectable text
• Fix Linux os.startfile() crash — linux_api.py used the Windows-only os.startfile(); replaced with xdg-open
• Add Linux/macOS clipboard support in cmd_client.py (pbcopy/xclip)

Test plan

• Tested on macOS — app launches, screenshot OCR, batch image OCR, batch document OCR (with text layer verified), system tray, hotkeys, HTTP API
• The PyMuPDF fix and Linux startfile fix are cross-platform corrections that benefit all platforms