Refactor billing repository interface and update usage

Renamed IBillingRepository to BillingRepository and updated all references across data, domain, and use case layers for consistency. Improved documentation and comments for repository and use cases. Adjusted typography letter spacing in design system and made minor UI tweaks to the spending breakdown card.

apps/mobile/packages/design_system/lib/src/ui_typography.dart

@@ -236,7 +236,7 @@ class UiTypography {
     fontWeight: FontWeight.w500,
     fontSize: 14,
     height: 1.5,
-    letterSpacing: 0.7,
+    letterSpacing: 0.4,
     color: UiColors.textPrimary,
   );
 
@@ -245,7 +245,7 @@ class UiTypography {
     fontWeight: FontWeight.w500,
     fontSize: 12,
     height: 1.5,
-    letterSpacing: 1.5,
+    letterSpacing: 0.7,
     color: UiColors.textPrimary,
   );
 
@@ -254,7 +254,7 @@ class UiTypography {
     fontWeight: FontWeight.w500,
     fontSize: 11,
     height: 1.5,
-    letterSpacing: 2.2,
+    letterSpacing: 0.7,
     color: UiColors.textPrimary,
   );
 
@@ -263,7 +263,7 @@ class UiTypography {
     fontWeight: FontWeight.w700,
     fontSize: 11,
     height: 1.5,
-    letterSpacing: 2.2,
+    letterSpacing: 0.7,
     color: UiColors.textPrimary,
   );