Add swipe-down aux screen (SOCKTOP_AUX_CMD)

A separate tmux window running any TUI, reached with a vertical swipe from
any carousel screen; swiping up returns to the exact window/zoom you left.
The aux window is excluded from the horizontal carousel and horizontal
swipes on it are inert. Bound only when SOCKTOP_AUX_CMD is set.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -16,6 +16,10 @@ left-to-right walks back the same way.
 
 With a single group this is simply overview -> each host -> back.
 
+Optionally, swiping **down** from any screen shows a different TUI (an
+[Uptime Kuma](https://github.com/louislam/uptime-kuma) status page on the rack
+display), and swiping **up** returns to exactly the screen you left.
+
 Built for a 10" touch panel on a server rack driving four Raspberry Pis, on a
 low-power x86 box running i3 on X11.
 
@@ -98,6 +102,8 @@ blame the config change.
 | --- | --- | --- |
 | `SOCKTOP_GROUPS` | Pis quartered `;` Orange Pi + Debian stacked | Groups separated by `;`, each a list of socktop profile names **in swipe order**, plus an optional `@layout` (`tiled` default, `even-vertical` stacks, `even-horizontal` side-by-side, `main-*` also work). Any count of groups and hosts works. |
 | `SOCKTOP_HOSTS` | empty | Legacy single-group form; used only when `SOCKTOP_GROUPS` is empty |
+| `SOCKTOP_AUX_CMD` | `uptime-kuma-status …` | Command for the swipe-down "aux" screen. Full path. Empty disables the feature and its gestures. |
+| `GESTURE_AUX_IN` / `GESTURE_AUX_OUT` | `UD` / `DU` | Swipe down shows aux, swipe up returns |
 | `SOCKTOP_SESSION` | `socktop4` | tmux session name |
 | `SOCKTOP_BIN` | `$HOME/.cargo/bin/socktop` | Full path — i3 does not have `~/.cargo/bin` on `PATH` |
 | `TOUCH_DEV` | ILITEK by-id path | Touch device. **Always use a `/dev/input/by-id/` path**; `eventN` numbers change on reboot. |