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/main.tsx

@@ -14,7 +14,6 @@ import {
   mountClosingBehavior,
   disableClosingConfirmation,
   mountBackButton,
-  mountMainButton,
   bindThemeParamsCssVars,
   bindViewportCssVars,
   requestFullscreen,
@@ -66,12 +65,6 @@ if (!alreadyInitialized) {
     } catch {
       /* already mounted */
     }
-    try {
-      mountMainButton();
-    } catch {
-      /* already mounted */
-    }
-
     // Viewport — async, fullscreen зависит от смонтированного viewport
     mountViewport()
       .then(() => {