feat(ui): add drag-and-drop image upload to composers

Add an ImageDropOverlay component and useImageDropTarget hook that let
users drop an image onto the home prompt card or canvas agent composer to
upload it as a reference, with a localized drop overlay.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/i18n/locales/en-US.json

@@ -58,6 +58,8 @@
   "logout": "Log out",
   "credits": "{count} Credits",
   "attachReference": "Attach reference",
+  "dropImageToUpload": "Release to upload image",
+  "dropImageSubtext": "Supports JPG, PNG, SVG or WebP",
   "webReference": "Web reference",
   "modelSelection": "Model selection",
   "webEnabled": "Web reference on",