Completes FC-6.2 with the UI.
- PostSeriesMenu: a "Series ▾" control on each post card — "New series from
this post" (promote → navigates to manage) and "Add to existing series…"
(dialog with a browsable picker loaded from GET /api/series, client-side
filtered — avoids the empty-autocomplete #712 trap).
- SeriesView (/series): a top-level Series browse grid — cover, name, artist,
chapter/page counts, gap badge; sort recent|name|size; cards → manage/read.
meta.title adds it to the nav automatically (peer of Posts).
- seriesBrowse store for the list.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
