refactor(base): SimpleLogin's forwarding page on the OpenID Connect module (ADR 0023)

halkyon_base depends on halkyon_oidc_server, and its forwarding page
is now the module's (`addOidcCallbackForwarding`) with Halkyon's table
of apps and its page in English or German; its tests keep what is
Halkyon's own, the rest is tested in the module.

Every host now includes the module: migrations `…-oidc-module` in
halkyon_base, Learn and Todo create its two tables (no data touched)
and record its version; Learn's devices get them empty, as they have
halkyon_base's server-only tables. halkyon_base_client depends on
halkyon_oidc_client, whose models its generated code uses, and Learn's
server image copies both packages into its narrowed workspace.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_base_server/pubspec.yaml

@@ -20,6 +20,10 @@ dependencies:
   # 0017): what a server needs of both, without libsodium. Pinned exactly, as
   # what keeps secrets must never move by accident.
   cryptography: 2.9.0
+  # Signing in with SimpleLogin (ADR 0020), on the OpenID Connect module
+  # (ADR 0023).
+  halkyon_oidc_server:
+    path: ../halkyon_oidc_server
   # Reads the pages profile links point to, for their rel="me" back-links.
   html: ^0.15.6
   # Asks the instance's register, and tells its members (ADR 0016).