feat(android): swipe between Erkunden sub-tabs (Bibliothek ↔ Global)

Wrap the Erkunden content in a HorizontalPager so a left/right swipe switches
between the Bibliothek and Global search tabs, two-way synced with the TabRow: a
chip tap animates the pager; a settled swipe updates the selected tab (and stops
any preview, like a tap). Removes the idea from the backlog now that it's built.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -453,7 +453,6 @@ Primary causes identified:
   - **New-release alerts:** periodic check for new albums/singles by followed artists (sources: iTunes lookup / MusicBrainz / Last.fm), surfaced in the existing in-app notification bell (and optionally push).
   - **Concert alerts:** reuse the existing **Ticketmaster** events integration to notify when a followed artist has nearby/upcoming shows.
   - Ties into the importer: a "download this new release" action straight from the notification.
-- **Android** — **Swipe between Erkunden sub-tabs** — a left/right swipe gesture on the Erkunden tab to switch between "Bibliothek" and "Global" search (instead of only tapping the chips). A fun, natural pager-style gesture (e.g. HorizontalPager).
 - **Android** — Playlist drag-and-drop / sort order
 - **Android** — Artist page iTunes fallback (for artists not in Navidrome)
 - **Android** — Romaji / Japanese sort support