Release 2.7.9: version bump + What's New / version modal + PR description + docker-publish default tag
- _SOULSYNC_BASE_VERSION -> 2.7.9; docker-publish default version_tag -> 2.7.9
- pr_description.md rewritten for 2.7.9 (best-quality downloads + quality profile #896, Discover listening recs + Listening Mix #913, Wing It Pool, Auto-Sync lane redesign, multi-disc #927, sync labels #925, post-processing race #928)
- WHATS_NEW: 2.7.8 block -> 2.7.9 (+ brief earlier-versions); VERSION_MODAL_SECTIONS: 2.7.9 highlights promoted, 2.7.8 rolled into an Earlier aggregator
- RELEASE_2.7.9_discord.md: mini Discord post
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