feat(authors): rebuild the page as an index and a gallery on one view toggle

The two layouts are the viewMode the app already persists, so search, filters
and sort carry across them. Only 19% of a library's authors have a portrait,
so the gallery leans on monograms and the index treats the name as the content;
a sort name now prints only when it says something the display name does not.
Jump buckets, a cover fallback and two filters come from the server so the A-Z
rail and the chips cover the whole result set rather than the loaded pages.

Reconciles authors.has_photo against the image store on boot: the list resolves
portraits from disk while the filter queried the column, so "no portrait" was
over-reporting by 64.

packages/types/src/author.ts

@@ -9,6 +9,18 @@ export type AuthorSummary = {
   imageUrl?: string | null;
   bookCount: number;
   lastAddedAt: string | null;
+  /**
+   * A book of theirs that has cover art, most recently added first. Only 19% of a
+   * typical library's authors have a portrait, so the grid falls back to this
+   * before it falls back to a monogram. Null when none of their books has a cover.
+   */
+  coverBookId?: number | null;
+};
+
+/** One bucket of the authors A-Z rail. `letter` is "#" for anything non-alphabetic. */
+export type AuthorLetterCount = {
+  letter: string;
+  count: number;
 };
 
 export type AuthorDetail = AuthorSummary & {

packages/types/src/display-preferences.ts

@@ -50,6 +50,9 @@ export interface DisplayPreferences {
   smartScopeFilterExpanded: boolean;
   authorCoverSize: number;
   authorCoverShape: AuthorCoverShape;
+  authorRowDensity: TableDensity;
+  /** Stand a book's cover in for a missing author portrait. Off: monogram instead. */
+  authorCoverFallback: boolean;
   tableZebraStriping: boolean;
   tableDensity: TableDensity;
   bookSpineOverlay: BookSpineOverlay;