* feat(desktop): drop files anywhere in the chat area
File drops were only wired to the composer input. Add a reusable
useFileDropZone hook (enter/leave depth counting + capture-phase reset so
the affordance clears even when the composer claims the drop) and a
pointer-events-none ChatDropOverlay, wired onto the conversation viewport.
Drops funnel through the existing onAttachDroppedItems; composer drops keep
their own inline-ref behavior.
* fix(desktop): chat-area drops insert inline @file refs, not attachment cards
Match the composer-input drop behavior — funnel dropped paths through
droppedFileInlineRef + the composer insert bus so they render as inline
ref chips instead of attachment cards.
* fix(desktop): don't render bare file paths as tool images (404)
vision_analyze reports its input image as a local filesystem path, which
toolImageUrl handed straight to <img src>. In the renderer that resolves
against the dev-server origin and 404s. Restrict inline tool images to
fetchable sources (data: URLs and remote http(s)); bare paths now fall
back to the tool's codicon.
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