feat(dnd): show clear landing position while dragging a block

[zhawtof]

Summary

While dragging a block from the palette, the user had no visual indication of where the new block would land. On top of that, dnd-kit's closestCenter collision detection was biased toward the tall surface droppable, so most palette drops landed at the end of the list regardless of where the cursor was aimed.

This PR makes the landing position obvious — both visually and behaviorally.

Changes

• Insertion bar: a 2px primary-colored line with a leading dot caret renders above the targeted row. When the cursor sits past the last block, the bar appears at the end of the list instead.
• Drop zone outline: the surface gets a dashed primary border + soft blue tint while a palette drag is in progress, with a deeper tint when the cursor crosses into it.
• Empty state: copy switches to "Drop to add block / Release to insert it here" in primary color, so the empty surface clearly reads as a drop target.
• Smarter collision detection: replaced closestCenter with a pointerWithin → closestCenter strategy that prefers the block under the cursor and only falls back to the surface for end-of-list drops.
• Reorder feedback unchanged: verticalListSortingStrategy's row shift is already a strong cue, so the new bar is gated on palette drags.

Side fix

demo/vite.config.ts now dedupes react/react-dom. Without it, the demo's alias to ../src/index.ts pulls a second copy of React from the workspace root and the page crashes with "Invalid hook call." This was needed to verify the change in the browser.

Test plan

• Typecheck (pnpm typecheck) passes
• Tests (pnpm test) — 34/34 pass
• Lint (pnpm lint) clean
• Drag a palette item over a block in the demo → indicator bar renders above that block; dropping inserts above it
• Drag a palette item past the last block → end-of-list bar renders; dropping appends
• Drag a palette item over an empty surface → "Drop to add block" copy and primary outline; dropping adds the first block
• Reorder existing blocks still works (sortable shift, no extra bar)
• Verify in dark theme

🤖 Generated with Claude Code