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>

halkyon_learn_server/pubspec.yaml

@@ -1,61 +0,0 @@
-name: halkyon_learn_server
-description: Starting point for a Serverpod server.
-# version: 1.0.0
-# homepage: https://www.example.com
-
-environment:
-  sdk: '^3.13.0'
-
-resolution: workspace
-
-dependencies:
-  clock: ^1.1.3
-  crypto: ^3.0.7
-  # What every Halkyon server has (ADR 0014): signing in, the account and its
-  # deletion, pruning, the instance's basics. A Serverpod module in
-  # halkyon_shared, pinned to a signed commit of its main; for local work on
-  # both, a git-ignored pubspec_overrides.yaml at the workspace root points it
-  # at a checkout.
-  halkyon_base_server:
-    git:
-      url: https://code.halkyon.app/project-halkyon/halkyon_shared.git
-      path: packages/halkyon_base_server
-      ref: 96f2d1461803599bdf5b14783f4b3d3a2810742f
-  # Reads the pages profile links point to, for their rel="me" back-links.
-  html: ^0.15.6
-  # Mails email sign-in's codes over SMTP (ADR 0010).
-  mailer: ^7.2.0
-  serverpod: 4.0.4
-  serverpod_auth_idp_server: 4.0.4
-  serverpod_offline_sync_server: 0.0.10
-dev_dependencies:
-  serverpod_test: 4.0.4
-  test: ^1.25.5
-
-serverpod:
-  # Companion Flutter apps that `serverpod start` can launch (Ctrl+R), keyed
-  # by a display alias. The default device is the web server, opening the
-  # machine's default browser when ready. All non-reserved properties are
-  # forwarded to `flutter run`. For the best IDE debugging experience, use
-  # `chrome` or a native target as the device. See the example below:
-  #
-  # Admin:
-  #   path: ../apps/admin_flutter
-  #   displayName: "Admin app"
-  #   auto_launch: true
-  #   device: chrome
-  #   target: lib/main.dart
-  #   web-hostname: 0.0.0.0
-  #   web-port: 8090
-  #   release: true
-  #   pub: false
-  #   dart-define:
-  #     API_URL=https://admin.example.com
-  #     FLAVOR=production
-  flutter_apps:
-    halkyon_learn:
-      path: ../halkyon_learn_flutter
-      displayName: "Halkyon learn app"
-      auto_launch: true
-      target: lib/driver.dart
-      device: linux