feat(tunein): add section-grouped results and load-more pagination

TuneIn's profiles API caps initial results at ~10 per container (Stations,
Shows, etc.) and exposes a Pivots.More.Url cursor for the remainder. This
change wires that cursor through the stack so users can load additional
results without leaving the search view.

- tuneInSearchSection now extracts Pivots.More.Url as bmx_next when
  itemToken is present; absent for containers already at their limit
- TuneInSearchNext fetches the cursor URL, which returns a flat Items[]
  (not nested containers), and maps Station/Program/Topic items using
  the existing play/profile builders
- New GET /v1/search/next and /api/tunein/search/next endpoints with
  matching handlers in both service paths
- TuneInBrowser: flat items state replaced with per-section sections
  state; each section shows a header label and a Load more button when
  a cursor is available; browse/navigate mode is unaffected

Relates to #336.

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

pkg/models/models.go

@@ -36,6 +36,7 @@ type Links struct {
 	BmxSearch               *Link `json:"bmx_search,omitempty" xml:"-"`
 	BmxPlayback             *Link `json:"bmx_playback,omitempty" xml:"-"`
 	BmxPreset               *Link `json:"bmx_preset,omitempty" xml:"-"`
+	BmxNext                 *Link `json:"bmx_next,omitempty" xml:"-"`
 }
 
 // BmxNavItem represents a single item in a TuneIn browse or search result.