Let the 3D mouse click all of SteamVR's Settings page

Most of SteamVR's Settings page took no clicks from the 3D mouse: they
went through to a desktop screen behind it, and the page covered the dot.
Steam's pages (Library and the rest) are drawn in
valve.steam.gamepadui.main, which ComputeOverlayIntersection hits exactly.
For SteamVR Settings that overlay is hidden and the page is drawn by the
dashboard's scene-graph panel, whose shape OpenVR doesn't give out. The
helper guessed a plane and a 0.5 m circle from the panel's transform, but
its origin is at the page's left edge and the page's surface is nearer
than that plane, so the laser, starting a few cm in front of the guess,
started behind the page.

On that page only (dashboard open, the main overlay hidden, the line of
sight crossing the page's measured area), the laser now starts 25 cm from
the eye so SteamVR's own hit test finds the page. The dot is drawn 0.6 m
out, in front of the page, and the laser-catching dot sits 8 m out,
invisible, with SteamVR's hit dot hidden on it. The beam and SteamVR's hit
dot show there, like a controller's; everywhere else nothing changes.

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

README.md

@@ -67,6 +67,7 @@ This is an early release, tested on one Steam Frame (SteamOS 0.3.0 build 2026092
 - Flatscreen games aren't detected as games. If your controllers end up working the screens instead of the game, set Controllers on the screens to "Only with the SteamVR dashboard open" (Frametop Display Settings, Visibility & wrist tab).
 - Typing follows your last click. A controller click on a panel other than the screens (the dashboard, a Steam app) doesn't move typing there; click it with the mouse, or click a screen to bring typing back.
 - The screens don't draw a mouse cursor of their own. The 3D mouse's dot or SteamVR's laser shows where you're pointing.
+- On SteamVR's Settings page, the 3D mouse shows a laser beam and a larger hit dot, like a controller. SteamVR doesn't tell other programs where that page is (unlike Steam's pages, such as Library), so the mouse used to miss most of it: clicks went through to a desktop screen behind, and the dot disappeared. As a workaround, on that page only, the laser starts near your eye and SteamVR finds the page itself. See docs/design.md.
 - Remote desktop over VNC (`./desktops.sh remote on`) needs Tailscale on the Frame.
 
 ## Reporting problems