Feat/refactor signals state

[Kimiti4]

Converted budget components from RxJS observables to Angular Signals.

• Benefits of Signals:
  • Fine-grained reactivity
  • Declarative and simpler than async pipe subscriptions
  • Computed values allow derived state

• Key changes:
  • SelectBudgetPageComponent uses inject(), signal(), computed(), and effect().
  • BudgetTableComponent now receives signals and reacts directly.

• Comparison with RxJS:
  • Signals are simpler for this UI scenario.
  • RxJS is still better for complex async streams or multiple event sources.