chore: scaffold the Serverpod 4 build

Deviations from a stock `serverpod create`:
- Dropped Flutter web support, for two reasons:
  - Appflowy editor had issues in the past, potentially related to the
    Flutter-Web IME bridge. Upstream has also open bugs in relation to
that:
    - AppFlowy-IO/appflowy-editor#1029 [Bug][Web] Unwanted space on click
       and overall space bugs (open)
    - AppFlowy-IO/appflowy-editor#782 [Bug] Editor auto-scrolling when
       pressing space too quickly if placed inside a scrollable (open)
  - Offline-first is a required feature. serverpod_offline_sync's SQLite
    support in browsers, maybe via sqlite3.wasm + OPFS, is not
documented.
- SDK constraints raised to ^3.13.0, instead of ^3.12.2, so enjoy the
  newest language features.
- Pinned to Flutter 3.47.4 via FVM.

Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_client/doc/endpoint.md

@@ -0,0 +1,15 @@
+# Callable endpoints
+
+Each class contains callable methods that will call a method on the server side. These are normally defined in the `endpoint` directory in your server project. This client sends requests to these endpoints and returns the result.
+
+Example usage:
+
+```dart
+// How to use GreetingEndpoint.
+client.greeting.hello("world!");
+
+// Generic format.
+client.<endpoint>.<method>(...);
+```
+
+Please see the full official documentation [here](https://docs.serverpod.dev)