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/mitm/store.rs

@@ -60,6 +60,17 @@ pub struct PendingToolResult {
     pub result: serde_json::Value,
 }
 
+/// A pending image to inject via MITM into the Google API request.
+/// The LS doesn't forward images from our SendUserCascadeMessage proto,
+/// so we inject them directly at the MITM layer.
+#[derive(Debug, Clone)]
+pub struct PendingImage {
+    /// Base64-encoded image data (no prefix).
+    pub base64_data: String,
+    /// MIME type, e.g. "image/png".
+    pub mime_type: String,
+}
+
 /// Client-specified generation parameters for MITM injection.
 /// Set by API handlers, consumed by the MITM modify layer.
 #[derive(Debug, Clone, Default)]
@@ -137,6 +148,10 @@ pub struct MitmStore {
     // ── Grounding metadata capture ──────────────────────────────────────
     /// Captured grounding metadata from Google API responses (search results).
     captured_grounding: Arc<RwLock<Option<serde_json::Value>>>,
+
+    // ── Pending image for MITM injection ─────────────────────────────────
+    /// Image to inject into the next Google API request via MITM.
+    pending_image: Arc<RwLock<Option<PendingImage>>>,
 }
 
 /// Aggregate statistics across all intercepted traffic.
@@ -181,6 +196,7 @@ impl MitmStore {
             response_complete: Arc::new(AtomicBool::new(false)),
             generation_params: Arc::new(RwLock::new(None)),
             captured_grounding: Arc::new(RwLock::new(None)),
+            pending_image: Arc::new(RwLock::new(None)),
         }
     }
 
@@ -506,4 +522,16 @@ impl MitmStore {
     pub async fn peek_grounding(&self) -> Option<serde_json::Value> {
         self.captured_grounding.read().await.clone()
     }
+
+    // ── Pending image for MITM injection ─────────────────────────────────
+
+    /// Store a pending image for MITM injection.
+    pub async fn set_pending_image(&self, image: PendingImage) {
+        *self.pending_image.write().await = Some(image);
+    }
+
+    /// Take (consume) pending image for injection.
+    pub async fn take_pending_image(&self) -> Option<PendingImage> {
+        self.pending_image.write().await.take()
+    }
 }