Critical UI/UX Fixes for Signup Screen and Environment Update

## [Bug] Critical: App Fails to Build on Dart 3.x and Signup Screen Has Layout Overflow/Misalignment

### 📝 Description

The `Create Account` (Signup) screen currently exhibits a few critical issues affecting user experience, and Version confilt. This issue needs to be addressed before new contributors can easily set up the project and for a better user experience.

#### Key Problems Identified:

1. **Build Failure on Modern Dart:** The project fails to compile on modern Dart versions (`3.x`) due to an outdated SDK constraint in `pubspec.yaml`, which prevents new users from running the app easily.
2. **Layout Overflow & Scrolling Issue:** The current vertical layout uses excessive fixed spacing, forcing users to scroll down unnecessarily to reach the "Sign In" link.
3. **UI Alignment Issues (Overflow):** Input field labels (e.g., "Team ID") are misaligned and overflow when the user inputs text, leading to a poor user experience.
4. **Code Inconsistency:** The navigation link used to switch to the Login page is implemented using an Text button and in Sign up page it is a button which affects the UI .

### 🖼️ Visual Evidence

### 🛠️ Steps to Reproduce

1. Clone the repository.
2. Run `flutter pub get`. (Observe the SDK constraint error).
3. Manually bypass the constraint and run the app on a device or emulator.
4. Navigate to the `signup_screen` .
5. Observe the large vertical spacing, misalignment of the input fields, and the required vertical scroll to see the "Sign In" link.

### 💡 Proposed Solution

The issues can be resolved by:
1. Updating the Dart SDK constraint in `pubspec.yaml` to the recommended exclusive upper bound (e.g., `>=2.17.0 <4.0.0`).
2. Refactoring the layout to use flexible spacing, removing fixed height `SizedBox` widgets, and using proper padding.

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