style(ui): refine composer and image drop border radius and padding

Drive the drop-target border radius from --image-drop-radius, enlarge the
overlay radius, and widen the composer editor padding.

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

frontend/src/ui/components/CanvasAgentComposer/index.css

@@ -30,7 +30,7 @@
 .assistant-composer.floating-composer {
   margin: 0;
   border-color: #cbd2dc;
-  border-radius: 28px;
+  border-radius: var(--image-drop-radius);
   --image-drop-radius: 28px;
   background: rgba(255, 255, 255, 0.98);
   box-shadow: 0 18px 45px rgba(17, 24, 39, 0.14);
@@ -40,7 +40,7 @@
   width: 100%;
   min-height: 48px;
   max-height: 240px;
-  padding: 0 4px;
+  padding: 0 20px;
   overflow-x: hidden;
   overflow-y: auto;
   border: 0;
@@ -55,6 +55,11 @@
   scrollbar-width: none;
 }
 
+.assistant-composer .prompt-rich-editor.compact {
+  padding: 0 20px;
+  min-height: 48px;
+}
+
 .assistant-composer .prompt-rich-editor::-webkit-scrollbar {
   display: none;
 }

frontend/src/ui/components/ImageDropOverlay/index.css

@@ -2,7 +2,8 @@
   position: relative;
   isolation: isolate;
   transition: border-color 180ms ease, box-shadow 180ms ease, transform 180ms ease;
-  --image-drop-radius: 8px;
+  border-radius: var(--image-drop-radius);
+  --image-drop-radius: 40px;
 }
 
 .image-drop-target.is-image-dragging {