Upgrade Dart/Flutter SDK and Dependencies (#26)

## Description ✍️
This PR introduces the following updates:
- Adds a `.tool-versions` file to pin development environment tools:
  - `flutter 3.32.8`
  - `dart 3.8.1`
- Updates `pubspec.yaml` to bump key dependencies:
  - `intl` from `^0.19.0` → `^0.20.2`
  - `rxdart` from `^0.27.7` → `^0.28.0`
  - `collection` from `^1.18.0` → `^1.19.1`

## Overview 👀
These updates ensure compatibility with the latest Flutter SDK and help
maintain stability with newer versions of core packages.

No functional or visual changes are expected.

## How to Smoke?
1. Run `asdf install` (if using asdf) to apply the pinned versions.
2. Run `flutter pub get` to fetch updated dependencies.
3. Ensure the app builds and runs without issues using the new SDK and
package versions.

Author: rodiguif

.tool-versions

@@ -0,0 +1,2 @@
+flutter 3.32.8
+dart 3.8.1

pubspec.yaml

@@ -13,9 +13,9 @@ dependencies:
     sdk: flutter
   # Follows the intl version included in Flutter.
   # https://github.com/flutter/flutter/blob/84a1e904f44f9b0e9c4510138010edcc653163f8/packages/flutter_localizations/pubspec.yaml#L11
-  intl: ^0.19.0
-  rxdart: ^0.27.7
-  collection: ^1.18.0
+  intl: ^0.20.2
+  rxdart: ^0.28.0
+  collection: ^1.19.1
 
 dev_dependencies:
   flutter_test: