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/lib/locales/en.ts

@@ -412,6 +412,7 @@ export default {
   'composer.abort': 'Abort',
   'composer.switchRemote': 'Switch to remote mode',
   'composer.attach': 'Attach file',
+  'composer.dropToAttach': 'Drop to attach',
   'composer.send': 'Send',
   'composer.stop': 'Stop',
   'composer.voice': 'Voice assistant',