fix(styles): resolve design system audit violations

- Add --color-text-on-accent token to tokens.css
- Migrate all hardcoded color:#fff/#ffffff to var(--color-text-on-accent) across all module stylesheets
- Fix toggle thumb background (#fff -> var(--color-surface)) for dark mode
- Migrate hardcoded transition durations to token vars (--transition-fast/base/slow)

public/styles/budget.css

@@ -161,7 +161,7 @@
 .budget-bar-row__fill {
   height: 100%;
   border-radius: var(--radius-full);
-  transition: width 0.4s ease;
+  transition: width var(--transition-slow);
 }
 
 .budget-bar-row__fill--income   { background-color: var(--color-success); }
@@ -322,11 +322,11 @@
 
 .amount-type-btn--income.amount-type-btn--active {
   background-color: var(--color-success);
-  color: #ffffff;
+  color: var(--color-text-on-accent);
 }
 
 .amount-type-btn--expenses.amount-type-btn--active {
   background-color: var(--color-danger);
-  color: #ffffff;
+  color: var(--color-text-on-accent);
 }