Support dragging files/images onto the composer to attach them (#32)

Reuses the existing upload path unchanged: handleFileSelected's body is
now handleFile(file), called from both the file-input's onChange and a
new onDrop handler on the composer, so drag-and-drop and the "Attach a
file" button share the exact same size-check/branch-on-content-type/
error-surfacing logic rather than duplicating it.

Only the first dropped file, matching the existing single-attachment-
per-message limit. A dashed-border overlay appears while dragging over
the composer for discoverability; a nested dragenter/dragleave counter
keeps it from flickering as the drag crosses child element boundaries.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/Composer.css

@@ -1,4 +1,5 @@
 .composer {
+  position: relative;
   padding: var(--sp-3) var(--sp-4);
   border-top: 1px solid var(--ds-border);
   background: var(--ds-surface);
@@ -7,6 +8,22 @@
   gap: 6px;
 }
 
+.composer-drop-overlay {
+  position: absolute;
+  inset: 4px;
+  z-index: 20;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: color-mix(in srgb, var(--ds-accent) 12%, var(--ds-surface) 88%);
+  border: 2px dashed var(--ds-accent);
+  border-radius: var(--radius);
+  color: var(--ds-accent);
+  font-size: 0.86rem;
+  font-weight: 700;
+  pointer-events: none;
+}
+
 .composer-box {
   display: flex;
   gap: var(--sp-2);