feat(windows): server-side key anchoring

[stefanverleysen]

Summary

• Adds per-screen anchoredKeys option so specific keys stay on the server instead of being sent to clients
• Supports single keys (F13, Delete) and modifier combos (Alt+Tab, Ctrl+A)
• Combos use a pending-modifier approach so a modifier like Alt is held until the next key determines whether the combo should stay on the server or pass through

Changes

• Added anchoredKeys option to screen config (256-bit VK bitmask plus combo slots)
• Low-level keyboard hook intercepts anchored keys and combos before relay
• Config parsing and serialization for the anchoredKeys directive
• GUI text field added to the screen settings dialog
• Implemented server-side only so it works with any client platform

To test

1. Set up a Windows server with at least one client
2. Open screen settings for the server screen
3. Enter Alt+Tab in the "Anchored keys" field
4. Move the cursor to a client so focus switches
5. Press Alt+Tab. The server should switch windows instead of the client
6. Test single keys as well (for example F13). These should remain on the server