[Feature Request] Library organization: series support — display and interaction

**Note:** This issue covers the display and interaction for the series support proposed in #865.

**Suggested features — display and interaction**

**Display on the bookshelf**
Series could display similarly to folders, with a separate bookshelf series style setting letting users choose between overlap and 2x2 grid independently for series and folders, so they could look the same or different depending on preference. The series title and author could appear below the cover in small text as they do on individual book covers.

When displayed as a group, a small black badge in the bottom right corner of the cover could show the total book count by default, for example "5", switching to a read progress format once any books are read, for example "3/5" meaning three of five books read. When displayed as ungrouped individual books, the bottom right corner could optionally remain blank or show a small badge with the book's series number, for example "# 3", so where it belongs in the series is visible at a glance. A small colored icon in the position where the cloud indicator currently sits on individual books could distinguish series from folders at a glance — a series icon in one color, a folder icon in a contrasting color.

For the stack option, the cover shown could be either the first book or whichever book is currently being read or is up next, ideally a user setting.

**Interacting with the stack**
Tapping the stack could open a small popup consistent with how folders currently work, showing all books in the series in order with their covers (tap to read / long press to book details), the series title (tap to series details) and author (tap to view) at the top. Long pressing the stack itself could open the current or next unread book directly.

**Filtering and read status**
Series could use the same stack or 2x2 grid style during filtering. When a tag filter is active, a tag count badge could appear in the top left corner with a tag icon, for example '2 🏷', showing how many books in the series match the filter.

Tapping could open the popup showing all books in order, with non-matching books shown with a semi-transparent grey or red overlay rather than hidden entirely.

If set to 2x2 grid, during filtering covers could be prioritized: tagged and unread first, tagged and read second, untagged and unread third, untagged and read last.

**Sort order**
Series could appear at the top of the shelf by default sorted alphabetically by series title, with standalone books below. The existing sort menu could be expanded to include series-first, folders-first, individual books-first, or fully alphabetical mixed, or possibly allow users to set a ranked priority order. For example series first, then folders, then individual books, rather than choosing a single sort option.