polish(tui): improve layout responsiveness

[MiguelRegueiro]

Summary

• Use transparent TUI background rendering so the terminal background shows through.
• Keep progress labels readable without adding a filled empty-bar background.
• Make library columns resize more predictably, prioritizing titles and episode info over Last Seen.

Testing

• cargo fmt --all -- --check
• cargo test --locked
• cargo clippy --all-targets --all-features -- -D warnings
• Manual TUI resize smoke.