fix: app crashes when creating transaction with empty description

[MF-Rozi]

Describe the bug

When saving a transaction with an empty description in TransactionEntryPage, the application crashes with an unhandled UnexpectedValueError.

Steps to Reproduce

1. Open the new transaction screen (/transaction/new).
2. Select an amount and choose a Category (Envelope).
3. Leave the Description input blank.
4. Tap Save Transaction.
5. The application throws UnexpectedValueError on StringSingleLine.getOrCrash() during model conversion / entity instantiation and crashes.

Expected Behavior

• The app should not crash.
• If the description is left blank, the system should automatically generate a contextual description based on the transaction type and category name (e.g., Spending for Dining, Income from Salary, Investment in Stocks).
• Users should be able to quickly log transactions without being strictly forced to type a custom description every time.

Proposed Fix

• In TransactionCubit.submitTransaction(), evaluate if state.description.trim().isEmpty.
• Dynamically build a fallback description:
  • Expense: Spending for <Category>
  • Income: Income from <Category>
  • Investment: Investment in <Category>
  • Transfer: Transfer for <Category>
  • Fallback: Transaction
• Add unit tests in transaction_cubit_test.dart to ensure empty and whitespace descriptions never crash and correctly generate contextual defaults.