Add an AltGr key; full-mode letters flick to their AltGr symbol

The right alt in full mode is now AltGr: sticky like the other modifiers,
it types the keymap's third/fourth level (sk: AltGr+J = ') instead of an
Alt shortcut. In hold-keys mode it holds the physical right alt, since
KWin resets the modifier state after an ISO_Level3_Shift keysym.

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

layouts/template.json

@@ -109,7 +109,7 @@
                     {"type": "modifier", "modifier": "alt", "label": "alt", "width": 1.25},
                     {"type": "globe"},
                     {"type": "space", "width": 6, "stretch": true},
-                    {"type": "modifier", "modifier": "alt", "label": "alt", "width": 1.25},
+                    {"type": "modifier", "modifier": "altgr", "label": "alt gr", "width": 1.25},
                     {"type": "layer", "label": ".?123", "target": "numbers", "width": 1.25},
                     {"type": "dismiss"}
                 ]