Add album / playlist "download all" actions

[TheZupZup]

Overview

Today downloads are per-track. Letting users download a whole album or playlist would be convenient — but it must stay true to Linthra's "no surprise downloads" promise.

What needs to be done

• Add a "Download all" action for albums and playlists
• Show a confirmation before starting a bulk download
• Respect the existing cache size limit
• Respect the Wi-Fi-only / mobile-data setting
• Honour "Keep offline" / pinned behaviour so bulk downloads aren't silently evicted
• Guard against accidental full-library downloads

Goal

Users can deliberately download an album or playlist for offline use, with no surprise or runaway downloads.

Notes

• Downloads must remain user-initiated and Wi-Fi-only by default — this is a core privacy/UX promise.
• See docs/offline-cache.md.

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Suggested labels: enhancement, privacy, ui-polish