fix(mac): release remote drag during touch teardown

[franekmat]

What changed

Centralize remote gesture cleanup and run it when:

• touch input is disabled
• the Android client disconnects
• the Mac server stops
• a one-finger gesture transitions to two fingers

If SideScreen previously posted a synthetic leftMouseDown for a long-press drag, cleanup now posts the matching leftMouseUp. It also cancels pending long-press and momentum-scroll work before returning the gesture state to idle.

Why

The long-press drag path posts leftMouseDown when movement begins. Some interruption paths previously discarded the .dragging state without posting leftMouseUp.

One reproducible case is placing a second finger while dragging: Android sends a two-pointer down event, and the Mac host immediately replaced .dragging with the two-finger state. The active application could then continue behaving as though the left mouse button was held.

The same imbalance was possible if touch was disabled, the client disconnected, or the server stopped during a drag.

Validation

• swift test — 35 tests passed
• Clean ARM64 release build
• No Android or wire-protocol changes

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