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/thinking_rectifier.rs

@@ -251,6 +251,8 @@ mod tests {
             enabled: true,
             request_thinking_signature: true,
             request_thinking_budget: true,
+            request_media_fallback: true,
+            request_media_heuristic: true,
         }
     }
 
@@ -259,6 +261,8 @@ mod tests {
             enabled: true,
             request_thinking_signature: false,
             request_thinking_budget: false,
+            request_media_fallback: true,
+            request_media_heuristic: true,
         }
     }
 
@@ -267,6 +271,8 @@ mod tests {
             enabled: false,
             request_thinking_signature: true,
             request_thinking_budget: true,
+            request_media_fallback: true,
+            request_media_heuristic: true,
         }
     }