Fix gb review issues: _isLoadingMore stuck on refresh, empty footer space, pagination tests

- Reset _isLoadingMore = false inside _load()'s setState in both
  MediaGridScreen and PodcastEpisodesScreen, so a generation-mismatch
  early return in an in-flight _loadMore does not leave the spinner
  permanently stuck after a pull-to-refresh (major bug fix).
- Replace the empty Text('') with SizedBox.shrink() in
  _buildFooterSliver when hasMore=true and isLoadingMore=false to
  eliminate the 32px dead space (nit fix).
- Add three pagination widget tests to each screen: _loadMore appends
  a second page, _loadMore is a no-op while already in-flight, and
  pull-to-refresh while _loadMore is in-flight leaves _isLoadingMore=false
  after _load completes (regression coverage for the major bug).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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