Optional cover art preview in TUI when terminal supports images

[frznn]

Summary

Add an optional cover art preview to the TUI.

This should be a best-effort feature:

• if the terminal supports image rendering, show the cover art preview
• if it does not, keep the current text-only behavior
• the app must continue to work normally in both cases

Motivation

The app already downloads and saves cover art, but there is no visual confirmation inside the TUI. A small preview would improve metadata review without making image rendering a hard dependency.

Proposed behavior

• Add a setting such as:
  • Cover art preview: auto | off
• auto:
  • attempt to render a cover preview only when the terminal/backend supports it
  • otherwise silently fall back to the existing text status
• off:
  • never attempt preview rendering

Requirements

• No regression in terminals without graphics support
• No crash or broken layout when preview is unavailable
• Fallback remains the current text-based cover-art status
• The preview must be optional, not required

Notes

• Textual does not provide built-in portable image support, so this likely needs a third-party path such as textual-image or rich-pixels
• Terminal compatibility is expected to vary
• This should be treated as a progressive enhancement, not a core feature

Acceptance criteria

• A setting exists to enable/disable cover preview
• When enabled and supported, cover art is shown in the TUI
• When unsupported, the UI falls back cleanly to text-only status
• Existing cover-art download flow continues to work unchanged