feat: autostart as a service + separate password setup (set-password.bat)

- set-password.bat: one-time password setup -> writes webvnc.conf (HASH + VNC_PASSWORD), syncs UltraVNC via ensure-vnc-password.ps1; if the 'web-vnc' task is installed, restarts it (elevated) to reload the new password, skipping restart when the password is unchanged.
- run.bat: no longer prompts for the password; reads it from webvnc.conf (asks to run set-password.bat first if absent). Also clears --spawn-command (not just --spawn) when 5900 is already listening, to avoid spawning a 2nd VNC server.
- scripts/autostart-run.bat: non-interactive launcher for the scheduled task (reads webvnc.conf, finds only a LOCAL VNC server, never downloads at boot).
- scripts/install-autostart.bat / uninstall-autostart.bat: register/remove a 'web-vnc' scheduled task (ONSTART, SYSTEM, HIGHEST) via schtasks.
- scripts/restart-autostart.bat: restart the task to apply a new password; checks whether web-vnc.exe is running (stop+start, or just start).
- .gitignore: ignore webvnc.conf (secret).
- AGENTS.md / README.md: document the autostart flow, set-password.bat, the new scripts, and that the running gateway must be restarted to apply a password change.

.gitignore

@@ -31,4 +31,7 @@ internal/server/static/utils/
 internal/server/static/novnc-original.html
 
 # Downloaded VNC server (installed via scripts/get-vnc.ps1)
-/vnc/
+/vnc/
+
+# Stored web access password + hash (created by set-password.bat)
+webvnc.conf