chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

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_client/pubspec.yaml

@@ -0,0 +1,18 @@
+name: halkyon_learn_client
+description: Starting point for a Serverpod client.
+
+environment:
+  sdk: '^3.13.0'
+
+resolution: workspace
+
+dependencies:
+  # The client side of halkyon_base (ADR 0014), as the server's.
+  halkyon_base_client:
+    git:
+      url: https://code.halkyon.app/project-halkyon/halkyon_shared.git
+      path: packages/halkyon_base_client
+      ref: 96f2d1461803599bdf5b14783f4b3d3a2810742f
+  serverpod_auth_idp_client: 4.0.4
+  serverpod_client: 4.0.4
+  serverpod_offline_sync_client: 0.0.10