fix: align Flutter version config and document requirements (#2717)

- Change .fvmrc from "beta" to "stable" to match CI behavior
- Update pubspec.yaml Flutter constraint from ^3.38.0 to ^3.41.0
  to match what pubspec.lock actually requires
- Document Flutter version requirements in docs/setting_dev_env.md

Closes lichess-org/mobile#2716

.fvmrc

@@ -1,4 +1,4 @@
 {
-  "flutter": "beta",
+  "flutter": "stable",
   "flavors": {}
 }

docs/setting_dev_env.md

@@ -6,11 +6,12 @@ If you get stuck during the installation process the most suitable place to seek
 
 ## Installing Flutter
 
-This project uses Flutter.
+This project uses the latest **Flutter stable** release. The minimum required version is defined in `pubspec.yaml`.
 
 1. Follow [the flutter install guide](https://docs.flutter.dev/get-started/install).
    This project is meant to run on iOS and Android, so you need to follow the "Platform setup" section of that guide to
    install the iOS and/or Android platform.
+   Make sure to install the latest Flutter stable release.
 
 > [!WARNING]
 > Installing on Linux using `snapd` might cause some [problems](https://github.com/lichess-org/mobile/issues/123) building stockfish.

pubspec.yaml

@@ -7,7 +7,7 @@ version: 0.22.0+002200 # See README.md for details about versioning
 environment:
   # this should be updated with every new flutter stable release
   sdk: ^3.10.0
-  flutter: ^3.38.0
+  flutter: ^3.41.0
 
 dependencies:
   app_links: ^7.0.0