feat(mac): Pen / Draw Mode — single pointer draws instead of scrolling

[crsxmd]

Summary

Adds Pen / Draw Mode, a Touch Control toggle that routes a single pointer straight onto the left mouse button so a stylus draws instead of scrolling.

Closes #45.

The gesture state machine currently treats one pointer as a trackpad: a stroke that travels further than tapMaxDistance before longPressTime elapses becomes a scroll, so drawing pans the canvas. Waiting out the 500 ms long press before every stroke is the only way to get a line today, which is unusable for drawing.

With the toggle on, and for a single pointer only:

• touch down → leftMouseDown immediately, no long-press wait
• touch move → leftMouseDragged
• touch up → leftMouseUp (with mouseEventClickState set, so a tap still reads as a complete click)

Two pointers are dispatched before the mode is consulted, so two-finger scroll and pinch are untouched and panning/zooming a canvas still works.

Off by default. When the toggle is off the only changes on the existing path are one Bool read and two branches that are unreachable unless the mode has been enabled, so every current gesture behaves exactly as before.

This is direct-pointer input only. The touch payload carries x, y, action, pointerCount and no pressure or tilt, so this deliberately does not close out the planned full stylus support — it is the small, Mac-only half of #45 that needs no wire-protocol or Android change.

Relationship to open PRs

• Add stylus pressure, tilt, and hover support #33 (stylus pressure/tilt/hover) is the larger, more complete answer and changes the wire protocol and the Android client. This PR is not a competitor: if Add stylus pressure, tilt, and hover support #33 lands, this toggle can be dropped or folded into it. It exists because it is 2 files, Mac-only, and gives [Feature] Synthesize touch drag as mouse drag – enable stylus/finger drawing #45 a working stroke today.
• fix(mac): release remote drag during touch teardown #46 (release remote drag during touch teardown) overlaps this PR's teardown fix — we independently hit the same stuck-button problem and centralised the same four paths. Happy to rebase on top of fix(mac): release remote drag during touch teardown #46 and drop the duplicate part if fix(mac): release remote drag during touch teardown #46 goes in first; the pen path cannot ship without that fix, since it makes a held button the normal state of every stroke rather than a rare long-press case.

Also fixed

releaseHeldMouseButtonIfNeeded() posts the matching leftMouseUp whenever a stroke can end without its touch-up: client disconnect, stopServer(), touch input switched off, a second finger landing mid-stroke, and a fresh pen down that finds a previous stroke still open. Without it, pulling the USB cable mid-stroke leaves the left button physically down and every later cursor move drags across the Mac until a real mouse is clicked.

It guards on .penDrawing || .dragging, so it also covers the pre-existing long-press drag case (same ground as #46).

Known limitations

• No pressure or tilt (protocol carries neither) — see Add stylus pressure, tilt, and hover support #33.
• While the mode is on, one-finger scroll, momentum and tap-and-hold right click are unavailable. Tap and double tap still work. The Settings panel says so in an orange note.
• Stroke smoothness is bounded by the client: MainActivity runs one-finger moves through InputPredictor.predictPosition(12f) and reads only event.x/y rather than getHistorical*, so fast strokes can overshoot slightly at sharp corners. Left alone deliberately to keep this PR Mac-only.

Testing

Verified:

• swift build clean, no warnings
• scripts/build_mac.sh produces the universal bundle; installed and launched on macOS 26.5.2, Apple Silicon
• Toggle appears under Touch Control and persists across restarts (defaults read com.sidescreen.app SideScreen_penModeEnabled)
• Default-off path re-read line by line: no existing line removed or moved, all new code unreachable with the toggle off

Not yet verified, which is why this is a draft:

• Cable-pull mid-stroke actually leaving the button released

Verified on hardware (Xiaomi Pad 6, USB, macOS 26.5.2):

• Long continuous strokes draw as expected instead of scrolling
• Two-finger scroll and pinch still work with the mode on
• Double tap opens items (this needed the follow-up commit: a press declares its click count when posted, so pen mode has to decide at contact rather than on release)

Hardware here is a Xiaomi Pad 6 over USB. The reporter of #45 offered to test builds — happy to have this exercised on a Redmi Pad Pro before it is marked ready.

UI

The toggle sits under Touch Control directly below "Enable Touch Input", disabled when touch input is off, with a note listing what the mode turns off. Screenshot to follow before this leaves draft.

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