fix(navigation): make Telegram back button work on deep-link entry

Bot deep-links open the Mini App directly on nested routes (/admin,
/balance/top-up, /info, /profile, ...) where React Router history has a
single entry, so navigate(-1) was a no-op and the native back button
looked dead. Fall back to the derived parent route when there is no
in-app history (window.history.state.idx === 0).

src/utils/navigation.ts

@@ -0,0 +1,38 @@
+/**
+ * Navigation helpers for back-button behavior.
+ *
+ * The Telegram Mini App (and any deep-link entry) can be opened directly on a
+ * nested route — e.g. a bot button that opens `/admin` or `/balance/top-up`.
+ * In that case React Router's history stack holds a single entry, so a plain
+ * `navigate(-1)` has nothing to go back to and the back button appears dead.
+ * These helpers let callers detect that situation and fall back to a sensible
+ * parent route instead.
+ */
+
+/**
+ * React Router stores the current position in the history stack on
+ * `window.history.state.idx`. The first/entry record is `0`. When `idx === 0`
+ * there is no in-app history to go back to (the page is the deep-link entry).
+ */
+export function hasInAppHistory(): boolean {
+  const idx = (window.history.state as { idx?: number } | null)?.idx ?? 0;
+  return idx > 0;
+}
+
+/**
+ * Derive a parent route by dropping the last path segment.
+ *
+ *   /admin/users/123  → /admin/users
+ *   /balance/top-up   → /balance
+ *   /info             → /
+ *   /                 → /
+ *
+ * Used as the back-button target when there is no in-app history. If the
+ * derived path is not a real route the app's catch-all redirects to `/`,
+ * so the user is never left stuck.
+ */
+export function getFallbackParentPath(pathname: string): string {
+  const segments = pathname.replace(/\/+$/, '').split('/').filter(Boolean);
+  const parent = segments.slice(0, -1);
+  return parent.length ? '/' + parent.join('/') : '/';
+}