feat(learn): Explore by what a licence allows, a skeleton while the library loads, and an assistant's agreement per provider (ADR 0005, rule 12; ADR 0021, D66)

- Explore filters by what a studyset's licence lets a learner do: adapt
  it (no NoDerivatives) and use it commercially (no NonCommercial),
  which the instance reads from the release's licence
  (ExploreQuery.adaptable, commercial) and the app the same way
  (LicenseUse). An instance from before ignores both.
- While the studysets are read, the Library shows shimmering skeletons
  of their cards rather than a spinner: halkyon_ui's new Shimmer and
  SkeletonBox, still where the system asks for no animations.
- The scope's "Include private studysets" is offered only while the
  person has a private studyset.
- An agreement to what goes where is per provider (protocol and host):
  agreeing to one assistant agrees to every assistant of that provider,
  those granted later too, and the agreement sheet says so. No
  migration: it is written on every grant of the provider, so taking
  one back keeps it for the others, and read per provider from any of
  them.

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/learn/halkyon_learn_flutter/lib/l10n/app_localizations.dart

@@ -9711,6 +9711,48 @@ abstract class AppLocalizations {
   /// **'Only studysets holding each of these.'**
   String get exploreKindsIntro;
 
+  /// No description provided for @exploreLicense.
+  ///
+  /// In en, this message translates to:
+  /// **'Licence'**
+  String get exploreLicense;
+
+  /// No description provided for @exploreLicenseChosen.
+  ///
+  /// In en, this message translates to:
+  /// **'Licence: {count}'**
+  String exploreLicenseChosen(int count);
+
+  /// No description provided for @exploreLicenseIntro.
+  ///
+  /// In en, this message translates to:
+  /// **'Only studysets whose licence lets you do each of these. Everything here may be copied and passed on.'**
+  String get exploreLicenseIntro;
+
+  /// No description provided for @exploreLicenseAdapt.
+  ///
+  /// In en, this message translates to:
+  /// **'Adapt it'**
+  String get exploreLicenseAdapt;
+
+  /// No description provided for @exploreLicenseAdaptDetail.
+  ///
+  /// In en, this message translates to:
+  /// **'Change it and pass on what you made: no NoDerivatives licences.'**
+  String get exploreLicenseAdaptDetail;
+
+  /// No description provided for @exploreLicenseCommercial.
+  ///
+  /// In en, this message translates to:
+  /// **'Use it commercially'**
+  String get exploreLicenseCommercial;
+
+  /// No description provided for @exploreLicenseCommercialDetail.
+  ///
+  /// In en, this message translates to:
+  /// **'Use it for paid work or teaching: no NonCommercial licences.'**
+  String get exploreLicenseCommercialDetail;
+
   /// No description provided for @exploreClearFilters.
   ///
   /// In en, this message translates to: