Add image search prototype UI

TODO.md

@@ -266,6 +266,20 @@
   - backend debug broadcasts
 
 ## Recent Change Log
+- Date: `2026-03-24`
+- What changed:
+  - Added a frontend-only `Video / Image` media-type toggle to Zone A.
+  - Kept the existing backend-connected video search flow as the default mode.
+  - Added an image-search prototype panel with sample prompt chips and test images.
+  - Added mock image-result cards so the image-search layout can be reviewed before backend image search exists.
+- Why it changed:
+  - Image search is planned next, and the user wanted the Zone A UI shape in place first so the workflow can be tested before backend integration.
+- How it was verified:
+  - `node --check frontend/app.js`
+- What is still risky or incomplete:
+  - This is UI-only; image mode does not call a backend API yet.
+  - The test images currently use placeholder assets and do not represent final data contracts or modal behavior.
+
 - Date: `2026-03-24`
 - What changed:
   - Added an operating rule that future plans recorded in this repo should be written in Korean by default.