Fix TigerVNC password file generation

[Toetje585]

Summary

Fix TigerVNC password-file generation in run/nobody/start.sh.

The previous implementation piped responses into the interactive vncpasswd
command, suppressed its output, and did not verify that the expected password
file was created.

During testing, startup reported that password setup was proceeding, but
/home/nobody/.config/tigervnc/passwd did not exist. Xvnc then logged:

[info] Password length OK, proceeding to set password...
SVncAuth: Opening password file '/home/nobody/.config/tigervnc/passwd'
          failed

The updated password-generation path:

• Uses vncpasswd -f for non-interactive password generation.
• Writes the generated password to a temporary file first.
• Checks the vncpasswd exit status.
• Verifies that the generated file is non-empty.
• Applies mode 0600 to the password file.
• Atomically moves the completed file into place.
• Exits with a clear error when password generation fails.
• Does not log the supplied password.

Checklist

• Shell syntax validated with bash -n.
• Diff validated with git diff --check.
• Docker image rebuilt locally.
• Container started successfully.
• Password file created successfully.
• Password file ownership and permissions verified.

owner=nobody group=users mode=600 size=8 path=/home/nobody/.config/tigervnc/passwd

• Xvnc password-file error no longer present.