Add Steam screenshots, Tailscale remote access, VR-video fixes

- Screenshots: list the Frame's Steam screenshots, open them in the
  viewer, and save new ones to ~/Pictures/SteamFrame. Ids are validated
  before any shell, and copies land atomically.
- Tailscale: scripts/tailscale-on-frame.sh installs a userspace tailscaled
  as a lingering systemd --user service with no sudo, SHA-256 checked, safe
  to re-run, with --uninstall. docs/tailscale.md covers setup and warns that
  in userspace mode every Frame port, including loopback-only DevTools and
  ADB, is reachable from the tailnet.
- push-vr-video.sh: filenames starting with "-" are safe, symlinks are
  followed, and a real Videos\VR directory triggers a warning.
- Tests cover the screenshot routes (19 total).

Docs keep placeholder addresses for the headset and tailnet.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.claude/skills/steam-frame/SKILL.md

@@ -22,6 +22,7 @@ desktop or panels.
 | See the Frame from the Mac, or the Mac inside the Frame | `docs/streaming.md` | `scripts/run-on-frame.sh mac-screen` |
 | Files and clipboard | `docs/file-transfer.md` | `scripts/push.sh`, `scripts/paste-to-frame.sh` |
 | Android apps (Lepton) | `docs/apks.md` | `scripts/install-apk.sh` |
+| Reach the Frame off the home LAN (Tailscale) | `docs/tailscale.md` | `scripts/tailscale-on-frame.sh` |
 | Install or buy Steam games, Frame ratings | `docs/steam-games.md` | `ui/frame_steam.py` |
 | Flatpaks | `docs/streaming.md` | `scripts/install-apps.sh` |
 | Launch an app inside the desktop panel | the script's header comment | `scripts/run-on-frame.sh` |