Add media fallback rectifier for text-only models

Replace unsupported Anthropic image blocks with an [Unsupported Image] marker when routed models are text-only or upstream rejects image input.

Add rectifier settings for media fallback and heuristic model detection, wire the controls into the settings UI, and cover the sanitizer and forwarder gates with regression tests.

src-tauri/src/proxy/mod.rs

@@ -19,6 +19,7 @@ pub mod http_client;
 pub mod hyper_client;
 pub(crate) mod json_canonical;
 pub mod log_codes;
+pub mod media_sanitizer;
 pub mod model_mapper;
 pub mod provider_router;
 pub mod providers;