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>

session/frametop.conf.example

@@ -54,7 +54,6 @@ POINTER_PINCH_DEADZONE=1.5 # hands: degrees the pinching hand moves before the p
 POINTER_GRIP_GAIN=1        # hands: the pointer moves this times the gripping hand's angle
 POINTER_GRIP_BELOW=0.35    # hands: grips beginning more than this (m) below the eyes are ignored (hands on a desk)
 POINTER_PINCH_TYPING=1     # hands: no pinch within this many seconds of a key (typing touches thumb to index)
-META_DASHBOARD=0   # 1 = a Meta tap on a pass-through keyboard toggles the SteamVR dashboard (pointer mode only)
 SHARE_KEYS=0       # 1 = keys of keyboards grabbed for the desktop also go to @frametop_keys, for hotkey tools (any local process can listen)
 DISPLAY_OFF_MIN=0  # minutes without use before frametop-power turns the displays off, even if the headset seems worn (0 = never)
 DISPLAY_MOVE_MM=5  # displays off: the headset or a controller moving this far (mm) within 10 s counts as use