Document the required Flutter/Dart toolchain in README

Records the versions this app was built and verified with (Flutter 3.16.9,
Dart 3.2.6, Gradle 8.5, Android SDK 21/34) so a fresh clone does not have to
guess, and notes that pubspec.lock pins the working dependency set.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -15,6 +15,34 @@ For help getting started with Flutter development, view the
 [online documentation](https://docs.flutter.dev/), which offers tutorials,
 samples, guidance on mobile development, and a full API reference.
 
+## REQUIREMENTS
+
+This project was built and verified with the following toolchain:
+
+| Tool | Version |
+| --- | --- |
+| Flutter | 3.16.9 (stable) |
+| Dart | 3.2.6 |
+| Android compileSdk / targetSdk | 34 |
+| Android minSdk | 21 |
+| Gradle | 8.5 |
+| JDK | 17 (required by Gradle 8.5) |
+
+`pubspec.yaml` requires Dart `>=3.2.6 <4.0.0` and `pubspec.lock` requires
+Flutter `>=3.16.6`. Newer Flutter 3.x releases may work, but `pubspec.lock`
+pins the exact dependency versions that are known to build — do not run
+`flutter pub upgrade` unless you intend to re-verify the whole app.
+
+Check your local toolchain before building:
+
+$flutter --version
+
+To pin the exact version, either install the 3.16.9 SDK from the
+[Flutter release archive](https://docs.flutter.dev/release/archive), or manage
+it per-project with [fvm](https://fvm.app):
+
+$fvm use 3.16.9
+
 ## HOW TO BUILD
 
 Run the following from the project root (the directory containing `pubspec.yaml`):