feat: Add configurable image cache settings via theme.yaml

[TheNoumanDev]

Summary:

• Image caching is global - flutter_cache_manager uses a single shared cache for all images. Setting it per-widget would be misleading since the last value set would apply to all images anyway.

• App-level setting - Cache duration and size are app-wide decisions, not per-image decisions. Similar to how App.useSplashScreen or App.textScale work.

Example:

App:
  imageCache:
    stalePeriodMinutes: 10080  # default: 15
    maxObjects: 500            # default: 50