Polish the Lyrics tab UI when lyrics are unavailable

[TheZupZup]

Overview

The app already shows a basic message when LRCLIB returns no lyrics. This issue covers polishing that behaviour so the Lyrics tab always looks intentional and clean — not like something broke.

What's already done:

• A basic "Lyrics not found" message is displayed inline

What needs to be done

• Clear the Lyrics tab content between tracks so previous lyrics don't linger while a new track loads
• Show a consistent placeholder (e.g., "No lyrics available for this track") styled the same as regular lyrics text
• Distinguish between "no lyrics found" and "network error" — don't show a raw exception message to users
• Show a brief loading indicator while lyrics are being fetched in the background

Goal

Users always understand why lyrics aren't showing — the tab never looks broken or stale.

Related to: #73