Send typing to the panel clicked last

An ungrabbed keyboard reached both sides at once: gamescope, which in VR
reads every input device itself (the SteamOS build's InputStealer), typed
into its focused app, and ft-screens typed into the desktop. Space in the
desktop paused Spotify on the dashboard, including every space frame-voice
dictated. And while the SteamVR dashboard was open, the desktop got no
keys at all.

Typing now follows the last click. ft-screens sees clicks on its own
screens; the pointer helper reports the panel under the dot on each mouse
press, so a click on any other panel sends typing to Steam. While typing
goes to the desktop and the screens are showing, ft-screens tells the
relay every second, and the relay grabs pass-through keyboards. A grab
waits until no key is down, and the relay lets go if ft-screens stops
reporting. Controller clicks on other panels aren't visible to overlay
apps, so they don't move typing (noted in the README).

A program that reads every keyboard for a hotkey loses a grabbed one.
Repeating the keys on another input device doesn't work, since gamescope
reads that too, so with SHARE_KEYS=1 the relay sends them to
@frametop_keys as datagrams with the device name. It's off by default:
any local process that binds that name first would get every key typed
into the desktop.

The docs now say gamescope reads keyboards and the headset's buttons
itself; they said SteamVR passed keys on to it.

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

pointer/helper/ft-pointer.cpp

@@ -633,6 +633,9 @@ int main() {
                 continue;
             }
             if (std::strncmp(buf, "btn trigger 1", 13) == 0) {
+                // ft-screens sends the keyboard to the panel clicked last; it sees clicks on
+                // its own screens, but only we know when one lands on another panel.
+                SendTo(out, "ft_screens", "click " + (lastHit.empty() ? std::string("-") : lastHit));
                 leftHeld = true;
                 dragDistance = lastDistance;
                 tiltYaw = tiltPitch = 0;  // a new drag starts untilted