feat: replace payment modals with page-based navigation

- Add /balance/top-up route for payment method selection
- Add /balance/top-up/:methodId route for amount entry and payment
- Remove TopUpModal component (619 lines)
- Simplify InsufficientBalancePrompt to use navigate instead of modals
- Support ?amount and ?returnTo query params for cross-page data flow
- Fix nav isActive to highlight Balance on sub-routes
- Remove unused MainButton platform abstraction
- Increase toast accent opacity for better visibility

src/App.tsx

@@ -26,6 +26,8 @@ const Contests = lazy(() => import('./pages/Contests'));
 const Polls = lazy(() => import('./pages/Polls'));
 const Info = lazy(() => import('./pages/Info'));
 const Wheel = lazy(() => import('./pages/Wheel'));
+const TopUpMethodSelect = lazy(() => import('./pages/TopUpMethodSelect'));
+const TopUpAmount = lazy(() => import('./pages/TopUpAmount'));
 
 // Admin pages - lazy load (only for admins)
 const AdminPanel = lazy(() => import('./pages/AdminPanel'));
@@ -170,6 +172,26 @@ function App() {
             </ProtectedRoute>
           }
         />
+        <Route
+          path="/balance/top-up"
+          element={
+            <ProtectedRoute>
+              <LazyPage>
+                <TopUpMethodSelect />
+              </LazyPage>
+            </ProtectedRoute>
+          }
+        />
+        <Route
+          path="/balance/top-up/:methodId"
+          element={
+            <ProtectedRoute>
+              <LazyPage>
+                <TopUpAmount />
+              </LazyPage>
+            </ProtectedRoute>
+          }
+        />
         <Route
           path="/referral"
           element={