feat(meditate): the instance's library in the app: Explore, its player, courses, keeping offline

Explore shows the instance's library below Meditate's practices, where it
has one: its courses and items by category (Meditate's eleven, named in
English and German, and the operator's own) and by the same search; an
instance without one shows nothing more. An item's page: its recordings
to choose, Play, a favourite (kept in the sealed settings under
library/<id>), keeping it on the device and removing it, and for a sound,
a silent sitting with it looped; its credit and licence. The player
plays, pauses and seeks; played to its end it is kept as a sitting, and
as its course's lesson done (lesson_done, sealed and synced). Settings
lists the library's credits beside About; Today shows its favourites.

The last catalogue is kept in the person's folder for offline; files are
fetched with the catalogue's access, asked for again shortly before it
ends. ADR 0038, D16.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/meditate/halkyon_meditate_flutter/pubspec.yaml

@@ -43,6 +43,8 @@ dependencies:
   # Noto Sans and the widgets every screen is built of.
   halkyon_ui:
     path: ../../../packages/halkyon_ui
+  # The library's recordings fetched to keep on the device (ADR 0038, D16).
+  http: ^1.6.0
   # gen-l10n's output imports it.
   intl: ^0.20.3
   lucide_icons_flutter: ^3.1.20