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_server/analysis_options.yaml

@@ -1,5 +1,5 @@
 # The workspace's lint rules (see ../analysis_options.yaml), minus generated code.
-include: ../analysis_options.yaml
+include: ../../../analysis_options.yaml
 
 analyzer:
   exclude:

apps/learn/halkyon_learn_server/pubspec.yaml

@@ -12,15 +12,10 @@ 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.
+  # deletion, pruning, the instance's basics. A Serverpod module among the
+  # repository's shared packages.
   halkyon_base_server:
-    git:
-      url: https://code.halkyon.app/project-halkyon/halkyon_shared.git
-      path: packages/halkyon_base_server
-      ref: 96f2d1461803599bdf5b14783f4b3d3a2810742f
+    path: ../../../packages/halkyon_base_server
   # 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).