[audemed44]

Problem

In the book editor, a series is only assigned when you press the ✓ in the series row. Save Changes saved the text fields and silently dropped a series that was picked but not confirmed.

That was easy to miss, especially when adding from Z-Library, where the editor fills in the series by itself. Four books added that way today (Beneath the Dragoneye Moons 1–4) ended up in no series, even though the series was created. The live logs show POST /api/series and the PATCHes, but no POST /api/series/{id}/books/{book}.

Fix

• Save Changes now first adds the book to the series picked in the add row, with its number, then saves the rest.
• If adding the series fails, the editor stays open and shows the error.
• A row whose text doesn't match an existing series (nothing picked) is still ignored, so an unwanted Z-Library suggestion doesn't block saving.

Testing

• New Vitest case: add from Z-Library → series preselected → Save Changes sends POST /api/series/4/books/b1?sequence=2 before the PATCH. It fails on the old editor and passes with the fix.
• Frontend lint, prettier, tsc, all tests and the build pass.

[audemed44]

Folded into #51 as a new commit.