feat(ui): add drag-and-drop and file picker for image attachments

Add two intuitive ways to attach images to chat messages:
1. Drag-and-drop images onto the compose area (with visual feedback)
2. Click the paperclip button to open file picker

Existing clipboard paste functionality remains unchanged.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/styles/chat/layout.css

@@ -263,6 +263,43 @@
   cursor: not-allowed;
 }
 
+/* Drag-over visual feedback */
+.chat-compose--drag-over {
+  outline: 2px dashed var(--accent);
+  outline-offset: -2px;
+  background: rgba(var(--accent-rgb), 0.05);
+  border-radius: 12px;
+}
+
+/* File attach button */
+.chat-compose__attach {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 8px;
+  cursor: pointer;
+  flex-shrink: 0;
+  border-radius: 8px;
+  border: 1px solid var(--border);
+  background: transparent;
+  color: var(--muted);
+  transition: all 150ms ease-out;
+}
+
+.chat-compose__attach:hover {
+  background: var(--panel);
+  color: var(--text);
+  border-color: var(--border-strong);
+}
+
+.chat-compose__attach svg {
+  width: 18px;
+  height: 18px;
+  stroke: currentColor;
+  fill: none;
+  stroke-width: 1.5px;
+}
+
 .chat-compose__actions {
   flex-shrink: 0;
   display: flex;

ui/src/ui/views/chat.test.ts

@@ -93,4 +93,24 @@ describe("chat view", () => {
     expect(onNewSession).toHaveBeenCalledTimes(1);
     expect(container.textContent).not.toContain("Stop");
   });
+
+  it("renders file input for image attachments", () => {
+    const container = document.createElement("div");
+    render(
+      renderChat(
+        createProps({
+          onAttachmentsChange: vi.fn(),
+        }),
+      ),
+      container,
+    );
+
+    const fileInput = container.querySelector('input[type="file"]') as HTMLInputElement | null;
+    expect(fileInput).not.toBeNull();
+    expect(fileInput?.accept).toBe("image/*");
+    expect(fileInput?.multiple).toBe(true);
+
+    const attachLabel = container.querySelector('label[for="chat-file-input"]');
+    expect(attachLabel).not.toBeNull();
+  });
 });

ui/src/ui/views/chat.ts

@@ -108,10 +108,29 @@ function generateAttachmentId(): string {
   return `att-${Date.now()}-${Math.random().toString(36).slice(2, 9)}`;
 }
 
-function handlePaste(
-  e: ClipboardEvent,
-  props: ChatProps,
-) {
+function processFiles(files: FileList, props: ChatProps) {
+  if (!props.onAttachmentsChange) return;
+
+  for (let i = 0; i < files.length; i++) {
+    const file = files[i];
+    if (!file.type.startsWith("image/")) continue;
+
+    const reader = new FileReader();
+    reader.onload = () => {
+      const dataUrl = reader.result as string;
+      const newAttachment: ChatAttachment = {
+        id: generateAttachmentId(),
+        dataUrl,
+        mimeType: file.type,
+      };
+      const current = props.attachments ?? [];
+      props.onAttachmentsChange?.([...current, newAttachment]);
+    };
+    reader.readAsDataURL(file);
+  }
+}
+
+function handlePaste(e: ClipboardEvent, props: ChatProps) {
   const items = e.clipboardData?.items;
   if (!items || !props.onAttachmentsChange) return;
 
@@ -146,6 +165,34 @@ function handlePaste(
   }
 }
 
+function handleDragOver(e: DragEvent) {
+  e.preventDefault();
+  e.stopPropagation();
+  (e.currentTarget as HTMLElement).classList.add("chat-compose--drag-over");
+}
+
+function handleDragLeave(e: DragEvent) {
+  e.preventDefault();
+  e.stopPropagation();
+  (e.currentTarget as HTMLElement).classList.remove("chat-compose--drag-over");
+}
+
+function handleDrop(e: DragEvent, props: ChatProps) {
+  e.preventDefault();
+  e.stopPropagation();
+  (e.currentTarget as HTMLElement).classList.remove("chat-compose--drag-over");
+
+  const files = e.dataTransfer?.files;
+  if (!files || !props.onAttachmentsChange) return;
+  processFiles(files, props);
+}
+
+function handleFileSelect(e: Event, props: ChatProps) {
+  const input = e.target as HTMLInputElement;
+  if (input.files) processFiles(input.files, props);
+  input.value = ""; // Reset so same file can be selected again
+}
+
 function renderAttachmentPreview(props: ChatProps) {
   const attachments = props.attachments ?? [];
   if (attachments.length === 0) return nothing;
@@ -327,7 +374,12 @@ export function renderChat(props: ChatProps) {
           `
         : nothing}
 
-      <div class="chat-compose">
+      <div
+        class="chat-compose"
+        @dragover=${handleDragOver}
+        @dragleave=${handleDragLeave}
+        @drop=${(e: DragEvent) => handleDrop(e, props)}
+      >
         ${renderAttachmentPreview(props)}
         <div class="chat-compose__row">
           <label class="field chat-compose__field">
@@ -353,6 +405,22 @@ export function renderChat(props: ChatProps) {
               placeholder=${composePlaceholder}
             ></textarea>
           </label>
+          <input
+            type="file"
+            id="chat-file-input"
+            accept="image/*"
+            multiple
+            hidden
+            @change=${(e: Event) => handleFileSelect(e, props)}
+          />
+          <label
+            for="chat-file-input"
+            class="btn btn--icon chat-compose__attach"
+            title="Attach images"
+            aria-label="Attach images"
+          >
+            ${icons.paperclip}
+          </label>
           <div class="chat-compose__actions">
             <button
               class="btn"