Add playbar click-to-seek, fix search focus and cover art handling

[LargeModGames]

Summary

Three small backlog issues:

• Click or drag anywhere on the playbar progress bar to seek (@LargeModGames works great. Is it possible to add it to the progress bar to ? In addition to < > ? #157). The control buttons still take priority and the time label is not clickable.
• Pressing Enter in the search box now moves focus to the results instead of leaving it stuck in the input, including when you re-search from the results screen (A Way to Escape Search Box on Submit #191).
• A failed cover art fetch no longer raises a blocking error or aborts the now-playing update. It logs a warning and carries on (Ignore not able to load images #142).

Closes #157, #191, #142.

Testing

• cargo fmt --all
• cargo clippy --no-default-features --features telemetry -- -D warnings
• cargo test --no-default-features --features telemetry (231 passing, 5 new)
• cargo clippy --no-default-features --features "telemetry streaming cover-art" -- -D warnings (compiles the cover art and streaming code the slim build skips)

Additional notes

• @LargeModGames works great. Is it possible to add it to the progress bar to ? In addition to < > ? #157 adds a feature, so this is arguably a minor version bump rather than a patch. The changelog entries sit under an Unreleased heading for now.
• Seek mapping has a render-based test that checks the computed gauge start against the bar that actually gets drawn, alongside the click and drag behavior tests.