Use routing helpers in festival selector and preserve current tab

[Copilot]

The festival selector was using hard-coded navigation ('/${festival.id}') instead of the routing helper functions, and always navigated to the home tab regardless of where the user was.

Changes

• Replace hard-coded URL with buildFestivalHome() and buildFavoritesPath() helpers
• Preserve current tab when switching festivals - users on favorites stay on favorites
• Capture route state early before any mutations to avoid context invalidation
• Add debug logging for error cases (e.g., test environments)

Implementation

// Determine target based on current route
String targetPath = buildFestivalHome(festival.id);
try {
  final currentPath = GoRouterState.of(context).uri.path;
  if (currentPath.endsWith('/favorites')) {
    targetPath = buildFavoritesPath(festival.id);
  }
} catch (e) {
  debugPrint('Festival selector: Unable to get current route, using default: $e');
}
router?.go(targetPath);

This maintains user context when browsing different festivals and consolidates routing logic in the helper functions.

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