feat(web): drag-and-drop files onto chat panel to add as attachments (#936)

* feat(web): drag-and-drop files onto chat panel to add as attachments

Closes #935

Adds a `useDragOver` hook that detects when a file is being dragged over
the browser window and suppresses the browser's default file-open
behaviour for drops outside the accept zone.

A new `DragDropZone` component wraps the inner `AssistantRuntimeProvider`
content in `SessionChat`. It shows a semi-transparent overlay (dashed
border + "Drop to attach" label) on the right-side chat panel as soon as
any file drag is detected — regardless of where the pointer is on the
page. Dropping on the right panel adds the files as composer attachments
via the existing `api.composer().addAttachment()` path. Drops on the left
sidebar are suppressed (no navigation, no attachment).

via [HAPI](https://hapi.run)

Co-Authored-By: HAPI <noreply@hapi.run>

* fix(web): disable drag-drop zone when pendingSchedule is active

The backend rejects requests with both scheduledAt and attachments.
DragDropZone now respects pendingSchedule the same way paste and the
attach button do — disabled=true suppresses the overlay, sets
dropEffect='none', and skips addAttachment on drop.

via [HAPI](https://hapi.run)

Co-Authored-By: HAPI <noreply@hapi.run>

* fix(web): harden drag-drop default-action handling

Address HAPI Bot review on #936:

- useDragOver: cancel the browser's default file-open/navigation on the
  document-level `drop` event for file payloads, not only on `dragover`.
  Preventing default on `dragover` alone still lets the browser open a
  file dropped outside any zone (e.g. the sidebar), which could unload
  the app.
- DragDropZone: only preventDefault when the drop payload actually
  contains files, so non-file drops (e.g. dragging selected text into
  the composer) keep their default browser behaviour.

Add regression tests for both.

via [HAPI](https://hapi.run)

Co-Authored-By: HAPI <noreply@hapi.run>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(web): use Simplified Chinese for composer.dropToAttach in zh-CN

Address HAPI Bot review on #936: the new zh-CN string used Traditional
Chinese forms (放開以附加檔案) in the Simplified Chinese locale, which is
inconsistent with neighbouring keys (e.g. composer.attach = 添加文件).
Use 松开以添加文件 to match.

via [HAPI](https://hapi.run)

Co-Authored-By: HAPI <noreply@hapi.run>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: HAPI <noreply@hapi.run>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

web/src/components/SessionChat.tsx

@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useMemo, useRef, useState } from 'react'
 import { useNavigate } from '@tanstack/react-router'
 import { AssistantRuntimeProvider, useAssistantApi, useAssistantState } from '@assistant-ui/react'
+import { DragDropZone } from '@/components/AssistantChat/DragDropZone'
 import type { ApiClient } from '@/api/client'
 import type {
     AttachmentMetadata,
@@ -1137,7 +1138,8 @@ function SessionChatInner(props: SessionChatProps) {
 
             <AssistantRuntimeProvider runtime={runtime}>
                 <ShareSeedConsumer sessionId={props.session.id} sessionActive={props.session.active} />
-                <div className="relative flex min-h-0 flex-1 flex-col">
+                <DragDropZone disabled={sessionInactive || props.isSending || pendingSchedule != null}>
+
                     <HappyThread
                         // Key with prefix: different components under the same session
                         // (thread, scratchlist, composer) must have distinct keys to avoid
@@ -1337,7 +1339,7 @@ function SessionChatInner(props: SessionChatProps) {
                         sendError={props.sendError ?? null}
                         onClearSendError={props.onClearSendError}
                     />
-                </div>
+                </DragDropZone>
             </AssistantRuntimeProvider>
 
             {/* Voice session component - renders nothing but initializes voice backend */}