Gestures: flick, long-press alternates, trackpad, layer slide, rollover

- PointerTracker: per-finger gesture state machine (pure, unit-tested)
- AlternatesPicker + AlternatesPopup.qml: long-press choices, finger selects
- Space long-press trackpad moves the cursor with arrow keysyms
- Slide from .?123 onto a symbol types it and switches back
- New finger commits pending taps (iOS rollover)
- Flick animation: secondary symbol slides down as the finger moves
- Transparent popup margin with an input region that passes clicks through
- tst_keyboardsurface: end-to-end gesture tests on the real QML

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

qml/Theme.qml

@@ -13,6 +13,8 @@ QtObject {
     readonly property color text: dark ? "#ffffff" : "#000000"
     readonly property color secondaryText: dark ? "#a5a5aa" : "#8a8a8e"
     readonly property color handle: dark ? "#8e8e93" : "#7c7c80"
+    readonly property color accent: dark ? "#0a84ff" : "#007aff"
+    readonly property color accentText: "#ffffff"
 
     readonly property real windowRadius: 14
     readonly property real keyRadius: 6