Support ChatGPT desktop bundle and refresh anchored README captures

[ClaudiuSchuster]

Summary

• Discover the app-server binary bundled with the supported ChatGPT desktop app when no configured, PATH, or user-local Codex CLI is available, while preserving the existing CLI preference.
• Keep the initial setup usable when neither backend is installed, with native ChatGPT App and Codex CLI guidance.
• Polish the usage-menu headings and model-labelled history rows, and refresh the README's anchored Cinnamon captures with horizontal, vertical, Spark, four-ring, reset-dialog, and Codex-only 5h + 7d examples.

Validation

• make verify
• git diff --check
• All affected capture skills pass quick_validate.py.
• All isolated capture scripts pass bash -n and ShellCheck.
• Private Xvfb/D-Bus captures were visually inspected at original resolution; the host Cinnamon PID, panel settings, wallpaper, enabled applets, and desklets remained unchanged.

Notes

The README's general dual-model images intentionally show both model-specific panel indicators. The Codex-only two-ring and conditional four-ring images are explicitly labelled state examples.