Add trackpad mode for relative pointer input on touch devices

[benjaminfrombe]

Summary

noVNC maps touch input to absolute framebuffer coordinates, which is
awkward on phones/tablets: your finger covers the target, you can't
reach screen edges easily, and there is no way to zoom/pan locally.
This PR adds an opt-in trackpad mode that behaves like the iOS RDP
app or a laptop trackpad: one finger moves a virtual cursor relatively,
pinch zooms the view locally (without sending anything to the remote),
and two-finger drag either pans the magnified view or scrolls the remote.

What it does

Toggle via RFB.trackpadMode = true or the new "Trackpad mode (touch)"
checkbox in the settings panel.

One finger

• Moves the virtual cursor relatively (scaled by trackpadSensitivity,
  default 1.5) — driven from raw touchmove events so there is no dead
  zone before movement is visible
• Tap = left click at the virtual cursor
• Long-press = right click (and drag = drag with button held)
• Tap immediately followed by drag ("tap-and-a-half") = drag with left
  button held

Two fingers

• Pinch = local magnification (1×–5×) via a CSS transform on the canvas
  (GPU-composited, smooth, the remote is never touched)
• While magnified: two-finger drag pans the view toward the fingers
• At fit (1×): two-finger drag sends mouse-wheel scroll events
• Zoom is anchored at the midpoint between the fingers (RDP-style)

Cursor visibility

• A dot cursor is always shown in trackpad mode (falls back to the dot
  when the server sends no visible cursor), so you can always see where
  the virtual cursor is

iOS Safari

• Blocks the browser's own pinch-to-zoom (which ignores user-scalable=no
  since iOS 10) by preventing the proprietary gesturestart/gesturechange
  events (detected via their .scale property, so noVNC's own synthetic
  gesture CustomEvents are unaffected)

Bug fix included

gesturehandler.js _twoTouchTimeout: the existing code chose between
PINCH and TWODRAG by comparing finger movement per axis against the
inter-finger distance change. A diagonal drag splits its movement across
both axes (~70% each), so it would lose to distance noise and be
misclassified as PINCH even when the fingers stayed parallel. Fixed by
comparing the 2D movement magnitude (hypot) instead.

Implementation notes

• All two-finger and one-finger movement handling is in raw touchstart/ move/end handlers that run alongside the gesture handler. The gesture
  handler retains responsibility for tap classification only.
• The CSS transform (translate()+scale(), origin 0 0) is purely visual;
  it does not affect the canvas layout size, so absX/absY still map
  pointer coordinates correctly to framebuffer pixels.
• The virtual cursor position is kept in untransformed canvas CSS pixels;
  _trackpadUpdateCursor maps it through the bounding rect's effective
  scale so the dot follows the logical position even while magnified.
• trackpadMode and trackpadSensitivity are public properties on RFB.
  Leaving trackpad mode resets the CSS transform.

Test plan

• Enable "Trackpad mode (touch)" on a mobile browser and verify:
  • One-finger drag moves the cursor from the first pixel (no dead zone)
  • Tap clicks, two-finger tap right-clicks
  • Long-press holds the button; tap-then-drag does the same
  • Pinch zooms in/out around the midpoint between fingers
  • While zoomed: two-finger drag pans the view (including diagonally)
  • At fit: two-finger drag scrolls the remote
  • Dot cursor is visible without moving first
  • iOS Safari does not zoom the page on pinch
• Verify existing desktop behaviour is unaffected (trackpadMode defaults to false)
• npm test passes (654 tests)

🤖 Generated with Claude Code

[tobfah]

Hi, thanks for your contribution. The first impression is that the modularization of this new trackpad feature is lacking -- see for instance how keyboard and cursor are integrated with rfb.