Feat/refactor signals state

[maxine-mwanda]

What was done

• Refactored SelectBudgetPageComponent and BudgetTableComponent to use Angular Signals instead of RxJS Observables.
• Removed all subscriptions and SubSink usage.
• Added effect() to reactively update the table data.
• Added computed() to derive overview budgets.

Benefits of Signals over Observables in this UI

• Cleaner, more declarative code.
• Reduced boilerplate and less chance of memory leaks from forgotten subscriptions.

Difference in coding style

• Signals simplify state flow in component hierarchies, especially for parent → child data passing.