Adds a new configuration option `routeWithFallback` to CapacitorConfig.server that enables
fallback to index.html for routes containing dots when the requested file doesn't exist.
This solves the issue where SPA routes like /@user.name or /api/data.json are incorrectly
treated as file requests, returning 404 instead of serving index.html for client-side routing.
The feature is disabled by default to maintain backward compatibility. When enabled:
- iOS: Checks if the file exists before serving, falls back to index.html if not found
- Android: Catches IOException when file not found, serves index.html as fallback
Fixes ionic-team#8085
🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>

The previous approach of checking file existence in the Router was causing
frame load interruptions. This moves the fallback logic to the WebViewAssetHandler's
error handling, similar to the Android implementation.
When a file with an extension fails to load and routeWithFallback is enabled,
we catch the error and serve index.html instead, allowing SPA routes to work properly.
Fixes iOS routing issues for URLs with dots like /@user.name