Drop files as @-references; move internal DnD to pointer events

[Ed-Barnes937]

What

Dropping OS files onto the window now inserts each as an @<path> reference into the active session's prompt — reusing the file explorer's PTY-write path.

Getting file paths on drop requires Tauri's OS drag-drop handler (dragDropEnabled: true), but that handler swallows the webview's HTML5 drag events. So the four internal drag flows are rebuilt on pointer events.

Changes

• tauri.conf.json — dragDropEnabled: true.
• main.ts —
  • onDragDropEvent handler: writes @<path> per dropped file to the active session's PTY, then focuses the terminal.
  • A shared draggable(handle, begin) primitive: threshold-gated pointer drag with pointer capture, Esc/cancel handling, and click suppression. Commits on release over a target; Esc/pointercancel leaves state untouched (mirrors the old drop-vs-dragend split).
  • Converted all four flows and removed the now-dead dragging* module vars and the makeSectionDropTarget / makeCardDropTarget wiring (targets are now annotated with data attributes and hit-tested from the source).
• Tests — iwft drag helpers drive real pointer events (page.mouse) instead of synthetic DragEvents; added pointerDragElement to the base page object.

Flows converted

Flow
Tab reorder + tab→terminal split-drop	✅
Sidebar row → section header	✅
Board card → section column	✅
Board column reorder	✅

Verification

• npm run typecheck passes
• Unit: 49 passed
• iwft (real-browser pointer events): 95 passed
• File drop confirmed working manually on macOS / Tauri 2.11

Notes

• No OS drag-ghost image anymore (native dnd rendered one). Feedback is the dimmed source + existing insertion markers / column highlights.
• Dropped paths containing spaces won't resolve cleanly as @ references (Claude Code ends the ref at whitespace). Matches the file explorer's existing unquoted behavior; can add quoting if wanted.

🤖 Generated with Claude Code