Index group libraries, gated by an indexing-scope preference

[introfini]

Summary

ZotSeek's bulk "Update Library Index" only indexes My Library, while auto-index
(when enabled) already indexes items from any library, including groups. This is
inconsistent, and there is no way to control which libraries get indexed.

Add first-class support for indexing group libraries, controlled by a single
indexing-scope preference that both the bulk action and auto-index respect.

Current behavior

• Manual "Update Library Index" indexes only the user library
  (getLibraryItems() on userLibraryID).
• Auto-index (zotseek.autoIndex): shouldProcess() in
  src/core/auto-index-manager.ts applies no library filter, so new items in
  group libraries are already indexed as they arrive.
• The storage layer is already library-aware: everything is keyed by
  (library_key, item_key), and the search engine can filter by libraryId
  via store.getByLibrary().

So the only missing pieces are a bulk entry point for group libraries and user
control over scope.

Proposed behavior

Introduce a preference that decides which libraries ZotSeek indexes:

zotseek.indexScope = "user" (default) | "all" | "group:123,group:456"

• Default "user" (My Library only). Group content is opt-in, so upgrading does
  not silently start indexing shared group libraries.
• The preference governs BOTH paths:
  • onIndexLibrary() in src/index.ts (bulk): pick getAllLibraryItems() vs
    getLibraryItems() based on the preference.
  • shouldProcess() in src/core/auto-index-manager.ts: reject items whose
    libraryID is out of scope. Auto-index cannot prompt the user, which is why
    scope must be a persisted preference rather than a per-click menu choice.
• The value format is extensible on purpose: the v1 UI can be a simple
  "My Library only / All libraries" toggle, while a future per-library selector
  (group:123,group:456) needs no preference migration.

UI

A dropdown in the ZotSeek preferences pane, next to the indexing settings.
v1: My Library only (default) / All libraries.

Acceptance criteria

• New zotseek.indexScope preference, default "user", with a UI control.
• Bulk index and auto-index both honor it.
• Menu / confirm / resume strings reflect the active scope.
• Existing resume markers (type: 'library' scope) keep working.

Related

PR #40 adds the multi-library bulk plumbing (getAllLibraries(),
getAllLibraryItems(), MCP library_key) and fixes a group-collection resume
bug. It currently flips the default to index all libraries; this issue narrows
that to the opt-in preference described above.