Full keyboard mode, Ctrl/Alt/Super, dead keys, shift levels

- Full (iPad Pro) mode: every typing key from the keymap, number row,
  tab, caps lock, ctrl/super/alt; shifted symbol shown above and flickable
- Dead keys compose with the locale's XKB compose table
- Ctrl/Alt/Super stick like shift and turn keys into shortcuts; a tapped
  or held shift joins shortcuts and non-text keys, auto-shift and caps
  lock don't
- Auto-capitalization and caps lock only capitalize letters; the user's
  shift picks the keymap's shifted level
- Mode switch in settings keeps key size by resizing the window

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

qml/Key.qml

@@ -14,13 +14,26 @@ Item {
     required property real keyHeight
     required property bool pressed
     required property real flickProgress
+    required property string modifier
     property int shiftMode: KeyboardController.Off
+    property var modifierModes: ({})
     property bool blank: false // labels hidden, e.g. in trackpad mode
 
     readonly property bool isCharacter: keyType === "char" || keyType === "space"
-    readonly property bool shiftActive: keyType === "shift" && shiftMode !== KeyboardController.Off
+    // Off / Once / Locked for shift, caps lock and ctrl/alt/super.
+    readonly property int latch: {
+        if (keyType === "shift")
+            return shiftMode
+        if (keyType === "capslock")
+            return shiftMode === KeyboardController.Locked ? KeyboardController.Locked : KeyboardController.Off
+        if (keyType === "modifier")
+            return modifierModes[modifier] ?? KeyboardController.Off
+        return KeyboardController.Off
+    }
+    // Shift shows locking with its glyph; the others turn accent-coloured.
+    readonly property bool locked: latch === KeyboardController.Locked && keyType !== "shift"
     // Character keys darken when pressed; function keys lighten (as on iPad).
-    readonly property bool lit: (isCharacter !== pressed) || shiftActive
+    readonly property bool lit: (isCharacter !== pressed) || latch !== KeyboardController.Off
 
     readonly property var defaultGlyphs: ({
         backspace: "⌫",
@@ -51,7 +64,7 @@ Item {
         id: face
         anchors.fill: parent
         radius: Theme.keyRadius
-        color: key.lit ? Theme.characterKey : Theme.functionKey
+        color: key.locked ? Theme.accent : key.lit ? Theme.characterKey : Theme.functionKey
     }
 
     // Flicking slides the secondary symbol down into the main position.
@@ -81,7 +94,7 @@ Item {
             verticalCenterOffset: flick.visible ? key.height * 0.14 : 0
         }
         font.pixelSize: key.height * (key.isWord ? 0.24 : flick.visible ? 0.34 : 0.4)
-        color: Theme.text
+        color: key.locked ? Theme.accentText : Theme.text
         visible: !key.blank
         opacity: 1 - key.flickProgress
     }