Shortcuts: Steam menu, commands, and modifier taps

Key combinations (and mouse and controller buttons) get two new actions:
- steam_menu: Open Steam menu / close dashboard (steam/ft-steam menu).
- command:CMD: run CMD with sh -c, as the relay's service, with layout/,
  float/ and steam/ on its PATH. Input Settings offers it for key
  combinations as Run a command...
Both work without pointer mode.

A modifier on its own is now a key combination too: a tap, pressed and
released with no other key, mouse button, or scroll in between. A bound
tap sends the desktop F24 before the release, so Plasma's launcher stays
shut. The defaults gain a Meta tap for the Steam menu; this replaces
META_DASHBOARD, which only worked in pointer mode.

input/test/keys-test.py runs the relay against fake devices with every
outgoing socket renamed, so it's safe next to the live relay.

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

scripts/keys-report.py

@@ -112,7 +112,7 @@ def main():
     try:
         with open(os.path.join(HOME, ".config/frametop.conf")) as f:
             conf = [ln.split("#")[0].strip() for ln in f]
-        out("Settings: " + " ".join(c for c in conf if re.match(r"(POINTER|META_DASHBOARD|SHARE_KEYS|BACKEND)=", c)))
+        out("Settings: " + " ".join(c for c in conf if re.match(r"(POINTER|SHARE_KEYS|BACKEND)=", c)))
     except OSError:
         out("Settings: no ~/.config/frametop.conf")
     out("ft-screens state (visibility, manual, wrist, gesture, lasers, game running, in games): "