feat: inject images via MITM layer instead of relying on LS

The LS silently ignores the 'images' field from our
SendUserCascadeMessageRequest proto — it never forwards image data
to Google's API.

New approach: store the image in MitmStore, then the MITM request
modifier injects it as 'inlineData' directly into the last user
message's parts array in the Google API JSON request.

Flow:
  Client → Proxy (decode base64) → MitmStore.set_pending_image()
  LS → Google API → MITM intercepts → inject inlineData part
  → Google receives image + text together

This works for all three API endpoints (responses, completions,
gemini).

src/api/completions.rs

@@ -303,6 +303,16 @@ pub(crate) async fn handle_completions(
 
     // Send message on primary cascade
     state.mitm_store.set_active_cascade(&cascade_id).await;
+    // Store image for MITM injection (LS doesn't forward images to Google API)
+    if let Some(ref img) = image {
+        use base64::Engine;
+        state.mitm_store.set_pending_image(
+            crate::mitm::store::PendingImage {
+                base64_data: base64::engine::general_purpose::STANDARD.encode(&img.data),
+                mime_type: img.mime_type.clone(),
+            }
+        ).await;
+    }
     match state
         .backend
         .send_message_with_image(&cascade_id, &user_text, model.model_enum, image.as_ref())