Add Frametop Remote Access, a settings app for the VNC view

A GTK 4 / libadwaita app (remote/ft-remote-settings, on the host's own
Python like the gaze probe): remote access on and off (REMOTE, applied at
once when the desktop allows it), the tailnet name and address to connect
to, and the VNC password shown, copied, or replaced. The password stays
random and made on the Frame, in ~/.config/frametop-remote; none is in
the code.

session/remote-ctl.sh starts, stops, and reports remote access; the
session uses it and leaves a remote-capable marker, since KWin allows the
capture only in a desktop that started with REMOTE=1. The password
between krdp and the VNC bridge (local only, but on krdp's command line)
is now new at every start. The installer adds the app to the menu.

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

README.md

@@ -82,7 +82,7 @@ This is an early release, tested on one Steam Frame (SteamOS 0.3.0 build 2026092
 - 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.
+- Remote desktop over VNC (Frametop Remote Access in the app menu, or `./desktops.sh remote on`) needs Tailscale on the Frame. It shows the primary screen only. The app turns it on and off, shows the address, and shows, copies, or changes the VNC password. The password is made at random on the Frame and kept in `~/.config/frametop-remote` (only you can read it); VNC limits it to 8 characters, and the tailnet encrypts the connection. Turning it on in a desktop that started with it off takes a desktop restart.
 - Turning the displays off on a stand only turns their backlight off. SteamVR has no way for other programs to put the headset in standby, so tracking and rendering keep running, and the headset draws nearly its full power.
 
 ## Reporting problems