Bulk-select a channel's videos into a playlist; last card no longer hidden under the bottom bars

- Channel pages join the select-mode views: tap Select in the filter row, pick
  videos (or Select all — respects the filter), then + Queue / + Playlist.
  Delete is hidden there (nothing to delete on a channel).
- --bottom-chrome (the fixed bottom nav + mini player height) is now measured
  whenever the mini player shows/hides and on resize/rotation, not only while
  the section rail scrolls, and the list pane pads its end by it, so the last
  card scrolls fully clear with or without the mini player.

frontend/index.html

@@ -376,6 +376,7 @@
             </div>
             <div id="batchBar" class="batch-bar hidden">
               <span id="batchCount" class="batch-count">0 selected</span>
+              <button id="batchAllBtn" class="batch-btn">Select all</button>
               <button id="batchQueueBtn" class="batch-btn">＋ Queue</button>
               <button id="batchAddPlaylistBtn" class="batch-btn">＋ Playlist</button>
               <button id="batchDeleteBtn" class="batch-btn batch-danger">Delete</button>