feat(library): genre sliders, artist links, and navigation utils

- music landing: diverse per-genre album sliders (online counts /
  offline wide-probe fallback) and home-screen library shortcuts
- add ArtistLinks component and shared navigation/genreDiversity utils
- player/playback-mode refinements across Rust and frontend

src/lib/stores/music.ts

@@ -2,9 +2,17 @@
 // Powers the focused music landing: hero + horizontal sliders.
 // TRACES: UR-007, UR-034 | DR-007, DR-038, DR-039
 import { writable, derived } from "svelte/store";
-import type { MediaItem } from "$lib/api/types";
+import type { MediaItem, Genre } from "$lib/api/types";
 import { auth } from "./auth";
 import { excludePodcasts } from "$lib/utils/podcastFilter";
+import { selectDiverseGenres, sampleAcross } from "$lib/utils/genreDiversity";
+
+/** A single "by genre" row: the genre name plus the albums in it. */
+export interface GenreRow {
+  id: string;
+  name: string;
+  items: MediaItem[];
+}
 
 interface MusicState {
   // Albums grouped from recently played tracks (backend handles grouping).
@@ -15,6 +23,8 @@ interface MusicState {
   playlists: MediaItem[];
   // Albums the user has played but hasn't returned to in a while.
   rediscover: MediaItem[];
+  // One slider per genre (top genres by album count).
+  genreRows: GenreRow[];
   // Mix of recently played + rediscover, used for the hero banner.
   heroItems: MediaItem[];
   isLoading: boolean;
@@ -22,6 +32,11 @@ interface MusicState {
 }
 
 const SECTION_LIMIT = 16;
+// How many genre sliders to show, and how many genres to probe to find them.
+const MAX_GENRE_ROWS = 8;
+// Probe a wide pool so the diverse-selection step has genres from across the
+// whole (alphabetical) list to choose between, not just the first handful.
+const MAX_GENRES_PROBED = 40;
 
 function createMusicStore() {
   const initialState: MusicState = {
@@ -29,6 +44,7 @@ function createMusicStore() {
     newlyAdded: [],
     playlists: [],
     rediscover: [],
+    genreRows: [],
     heroItems: [],
     isLoading: false,
     error: null,
@@ -110,6 +126,10 @@ function createMusicStore() {
         heroItems,
         isLoading: false,
       }));
+
+      // Genre rows are secondary — load them after the main sections paint so
+      // the page isn't blocked on N per-genre queries.
+      loadGenreRows(libraryId);
     } catch (error) {
       const message = error instanceof Error ? error.message : "Failed to load music sections";
       update(s => ({ ...s, isLoading: false, error: message }));
@@ -117,6 +137,76 @@ function createMusicStore() {
     }
   }
 
+  /** Fetch up to SECTION_LIMIT albums for one genre as a slider row. */
+  async function loadGenreRow(libraryId: string, genre: Genre): Promise<GenreRow> {
+    const repo = auth.getRepository();
+    try {
+      const result = await repo.getItems(libraryId, {
+        includeItemTypes: ["MusicAlbum"],
+        genres: [genre.name],
+        sortBy: "SortName",
+        sortOrder: "Ascending",
+        recursive: true,
+        limit: SECTION_LIMIT,
+      });
+      // HACK: drop the "Podcasts" folder that lives in the music library.
+      return { id: genre.id, name: genre.name, items: excludePodcasts(result.items) };
+    } catch (e) {
+      console.warn(`Failed to load genre row "${genre.name}":`, e);
+      return { id: genre.id, name: genre.name, items: [] };
+    }
+  }
+
+  /**
+   * Build one slider per genre, showing albums in each. We pick a *diverse* set
+   * rather than just the most populous — otherwise a cluster of near-synonyms
+   * ("Rock", "Hard Rock", "Classic Rock", ...) crowds out musically distinct
+   * genres. See selectDiverseGenres. Album genres rarely carry community
+   * ratings, so we order each row by name.
+   *
+   * When the backend reports per-genre album counts (online), we rank and pick
+   * before fetching, so we only query albums for the genres we'll actually
+   * show. When counts are missing (offline), we fall back to probing a wide
+   * pool, dropping empties, then ranking by what came back.
+   */
+  async function loadGenreRows(libraryId: string) {
+    try {
+      const repo = auth.getRepository();
+      const genres = await repo.getGenres(libraryId);
+      if (genres.length === 0) return;
+
+      const hasCounts = genres.some(g => g.albumCount != null);
+
+      let genreRows: GenreRow[];
+      if (hasCounts) {
+        // Rank by reported count, pick a diverse subset, then fetch only those.
+        const ranked = [...genres].sort(
+          (a, b) => (b.albumCount ?? 0) - (a.albumCount ?? 0)
+        );
+        const chosen = selectDiverseGenres(ranked, MAX_GENRE_ROWS);
+        genreRows = (await Promise.all(chosen.map(g => loadGenreRow(libraryId, g)))).filter(
+          row => row.items.length > 0
+        );
+      } else {
+        // No counts (offline, or a server that ignores Fields=ItemCounts).
+        // The genre list is alphabetical, so probing the first N would only
+        // ever surface A-genres. Sample at an even stride across the whole
+        // list instead, so the probe pool spans A→Z; then drop empties, rank
+        // by what came back, and pick a diverse subset.
+        const probed = sampleAcross(genres, MAX_GENRES_PROBED);
+        const rows = await Promise.all(probed.map(g => loadGenreRow(libraryId, g)));
+        const populated = rows
+          .filter(row => row.items.length > 0)
+          .sort((a, b) => b.items.length - a.items.length);
+        genreRows = selectDiverseGenres(populated, MAX_GENRE_ROWS);
+      }
+
+      update(s => ({ ...s, genreRows }));
+    } catch (e) {
+      console.warn("Failed to load music genre rows:", e);
+    }
+  }
+
   function reset() {
     set(initialState);
   }