feat: add image support across all endpoints (responses, completions, gemini)

src/backend.rs

@@ -343,17 +343,29 @@ impl Backend {
     }
 
     /// SendUserCascadeMessage with binary protobuf body.
+    #[allow(dead_code)]
     pub async fn send_message(
         &self,
         cascade_id: &str,
         text: &str,
         model_enum: u32,
+    ) -> Result<(u16, Vec<u8>), String> {
+        self.send_message_with_image(cascade_id, text, model_enum, None).await
+    }
+
+    /// SendUserCascadeMessage with optional image attachment.
+    pub async fn send_message_with_image(
+        &self,
+        cascade_id: &str,
+        text: &str,
+        model_enum: u32,
+        image: Option<&crate::proto::ImageData>,
     ) -> Result<(u16, Vec<u8>), String> {
         let token = self.oauth_token().await;
         if token.is_empty() {
             return Err("No OAuth token available".to_string());
         }
-        let proto = crate::proto::build_request(cascade_id, text, &token, model_enum);
+        let proto = crate::proto::build_request_with_image(cascade_id, text, &token, model_enum, image);
         self.call_proto("SendUserCascadeMessage", proto).await
     }