Migrate web server to ESPAsyncWebServer

[Gobol]

This PR migrates the project from the synchronous WebServer API to ESPAsyncWebServer.

Improvements:

• move request-scoped state into an explicit compatibility context
• stop buffering multipart upload in RAM
• remove body collection from static asset and read-only routes
• add frontend null guards to avoid bootstrap failures when optional panels are missing

Changes:

• replace WebServer usage with AsyncWebServer
• add a compatibility layer in Web.h / Web.cpp
• update JsonResponse to use async response streams
• switch static file serving and backup/download responses to async responses
• update upload handlers for async callbacks
• add ESP Async WebServer and AsyncTCP to CI/release workflows

Validation:

• built successfully with arduino-cli for generic ESP32
• verified on hardware via HTTP OTA