[Feature] Expand webchat drag-and-drop upload hot zone to the whole chat area.

[wcqqq1214]

Feature Description

AstrBot already supports drag-and-drop file/image upload in the webchat — it was added in PR #4583 and shipped since v4.12.4. However, the drop zone is currently limited to the small input composer bar at the bottom, which makes the feature easy to miss and error-prone when dragging onto the message area.

Use Case

Current Behavior

• The drag-and-drop hot zone is only the input composer area (.input-area inside ChatInput.vue).
• Dragging a file/image onto the message list area does nothing — no overlay, no upload.
• The drop overlay only appears when hovering directly over the input bar.

Expected Behavior

• The entire right-side chat area of webchat (message list + composer) should accept drag-and-drop uploads.
• Dropping anywhere in that area shows the upload overlay and triggers the same upload flow (image → image upload; other files → file upload).
• The left sidebar (session list) stays unaffected.

Checklist

• I have searched the Issue list for related features and confirmed this feature idea has never been proposed.
• The feature I am suggesting is related to AstrBot itself, not to a specific plugin.
• I have read and agree to the project's Code of Conduct.
• (Optional) I am willing to submit a PR to help implement this feature.