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);

frontend/src/components/Composer.tsx

@@ -4,6 +4,7 @@ import {
   useRef,
   useState,
   type ChangeEvent,
+  type DragEvent,
   type FormEvent,
   type KeyboardEvent,
 } from 'react'
@@ -55,8 +56,14 @@ export function Composer({ roomId, roomName, members, disabled, onSend }: Compos
   const [maxUploadBytes, setMaxUploadBytes] = useState<number | null>(null)
   const [mentionQuery, setMentionQuery] = useState<MentionQuery | null>(null)
   const [mentionActiveIndex, setMentionActiveIndex] = useState(0)
+  const [dragActive, setDragActive] = useState(false)
   const textareaRef = useRef<HTMLTextAreaElement>(null)
   const fileInputRef = useRef<HTMLInputElement>(null)
+  // Counts nested dragenter/dragleave pairs (the overlay, the composer box,
+  // the textarea are all separate elements a drag passes over) so the
+  // highlight doesn't flicker off every time the pointer crosses a child
+  // element boundary -- only actually leaving the whole composer zeroes it.
+  const dragCounterRef = useRef(0)
   const online = useOnlineStatus()
 
   const mentionMatches = useMemo(() => {
@@ -148,11 +155,7 @@ export function Composer({ roomId, roomName, members, disabled, onSend }: Compos
     setMentionActiveIndex(0)
   }
 
-  async function handleFileSelected(e: ChangeEvent<HTMLInputElement>) {
-    const file = e.target.files?.[0]
-    e.target.value = ''
-    if (!file) return
-
+  async function handleFile(file: File) {
     setUploadError(null)
 
     if (maxUploadBytes !== null && file.size > maxUploadBytes) {
@@ -179,6 +182,44 @@ export function Composer({ roomId, roomName, members, disabled, onSend }: Compos
     }
   }
 
+  function handleFileSelected(e: ChangeEvent<HTMLInputElement>) {
+    const file = e.target.files?.[0]
+    e.target.value = ''
+    if (file) handleFile(file)
+  }
+
+  function handleDragEnter(e: DragEvent<HTMLDivElement>) {
+    e.preventDefault()
+    if (disabled) return
+    dragCounterRef.current++
+    setDragActive(true)
+  }
+
+  function handleDragLeave(e: DragEvent<HTMLDivElement>) {
+    e.preventDefault()
+    dragCounterRef.current = Math.max(0, dragCounterRef.current - 1)
+    if (dragCounterRef.current === 0) setDragActive(false)
+  }
+
+  function handleDragOver(e: DragEvent<HTMLDivElement>) {
+    // Required even though it does nothing else -- without preventDefault()
+    // here, the browser rejects the element as a drop target entirely and
+    // handleDrop never fires (it just navigates to/opens the dropped file).
+    e.preventDefault()
+  }
+
+  function handleDrop(e: DragEvent<HTMLDivElement>) {
+    e.preventDefault()
+    dragCounterRef.current = 0
+    setDragActive(false)
+    if (disabled) return
+    // Only the first dropped file, matching the existing single-attachment-
+    // per-message limit (the button-triggered file input isn't `multiple`
+    // either).
+    const file = e.dataTransfer.files?.[0]
+    if (file) handleFile(file)
+  }
+
   function removePendingImage() {
     setPendingImage((prev) => {
       if (prev) URL.revokeObjectURL(prev.previewUrl)
@@ -205,7 +246,18 @@ export function Composer({ roomId, roomName, members, disabled, onSend }: Compos
   }
 
   return (
-    <div className="composer">
+    <div
+      className="composer"
+      onDragEnter={handleDragEnter}
+      onDragLeave={handleDragLeave}
+      onDragOver={handleDragOver}
+      onDrop={handleDrop}
+    >
+      {dragActive && (
+        <div className="composer-drop-overlay">
+          <span>Drop to attach</span>
+        </div>
+      )}
       {pendingImage && (
         <div className="composer-attachment">
           <img src={pendingImage.previewUrl} alt="" className="composer-attachment-thumb" />