feat: add multi-sync input mode

[Jackyzhangyi]

Fixes #216

Summary

Adds a minimal Multi-Sync Input mode (Altiris-style) for the desktop client.
When enabled, keyboard and mouse input from the active tab is replicated to all other open VNC connections.

What’s included

• Machine → Multi-Sync Input (checkable) with shortcut Ctrl+Shift+M
• Mouse replication with coordinate scaling for different framebuffer sizes
• Key replication (down/up/char)
• Releases modifier keys on focus loss; auto-disables Multi-Sync Input when the window loses focus

Manual QA

1. Open 2+ VNC connections (tabs).
2. On the active tab, enable Machine → Multi-Sync Input (Ctrl+Shift+M).
3. Move mouse / click / type on the active tab → verify actions replicate to other tabs.
4. Switch active tab → verify replication follows the active tab.
5. Alt-tab away / lose focus → verify Multi-Sync Input disables and modifiers are released.

Notes / limitations

• Local build not run in this environment (missing C++ toolchain: nmake/ninja/compiler). Please rely on CI / maintainer build.

Checklist

• Minimal scope change
• No refactors
• Manual QA steps provided

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #216: multi-sync input mode (similar to altiris)

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[Jackyzhangyi]

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