fix(08-02): revert repeat() inside lit-virtualizer, restore .renderItem + .keyFunction

repeat() as child content of lit-virtualizer bypasses virtualization,
rendering ALL items instead of only visible ones. This caused 2+ minute
loading times and UI freezing with large libraries.

The correct approach: use lit-virtualizer's native .renderItem and
.keyFunction properties which integrate with its scroll-based viewport
management.

frontend/src/components/track-list/track-list.ts

@@ -26,7 +26,6 @@ import {
     DEFAULT_COLUMN_IDS,
 } from './columns';
 import type { ColumnDef } from './columns';
-import { repeat } from 'lit/directives/repeat.js';
 import { classMap } from 'lit/directives/class-map.js';
 import {
     rankTracks,
@@ -1738,12 +1737,10 @@ export class TrackList extends LitElement implements SelectionHost, ContextMenuH
                       <lit-virtualizer
                         scroller
                         .items=${visibleTracks}
+                        .renderItem=${this.renderTrackRow}
+                        .keyFunction=${(track: library.Track) => track.FilePath}
                         .layout=${this.flowLayout}
-                      >${repeat(
-                          visibleTracks,
-                          (track) => track.FilePath,
-                          (track, index) => this.renderTrackRow(track, index),
-                      )}</lit-virtualizer>
+                      ></lit-virtualizer>
                     `}
 
       ${this.searchCtrl.term && visibleTracks.length > 0