Refactor resume to session queue model with autosave and takeover lock

[pythrick]

This replaces the earlier resume implementation with a session queue model and lock-based single-instance handling.

What changed:

• Store the current queue in ~/.config/cliamp/session_queue.toml
• Store lightweight resume metadata in ~/.config/cliamp/resume.json (current_index, position_sec)
• Restore session queue + index + position on startup (when no positional args are passed)
• Autosave session changes with a short debounce and periodic position updates while playing
• Keep shuffle/repeat as user config preferences (not session metadata)
• Add single-instance session lock to prevent concurrent resume file races
• Add --takeover to terminate the running instance and acquire the lock

Fixes #104