build: one workspace for every app and the packages they share

The root pubspec lists Learn's, Todo's and the shared packages, which
the apps now take by path instead of pinned commits of halkyon_shared:
one resolution, one lockfile. Learn's versions stay as they were; Todo
moves to Serverpod 4.0.4 and Dart 3.13 with the rest (its generated code
unchanged), drops google_fonts, which it never used, and a template
line Dart 3.13 refuses.

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

@@ -9,10 +9,7 @@ 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
+    path: ../../../packages/halkyon_base_client
   serverpod_auth_idp_client: 4.0.4
   serverpod_client: 4.0.4
   serverpod_offline_sync_client: 0.0.10