Fix: connectors honor the side you drag from

Side connectors (left/right/bottom) snapped to the top of the target card instead of attaching where you dragged. onConnect was discarding the sourceHandle/targetHandle from the connection, so React Flow fell back to the first handle (t, top). Connections now attach to the exact side, matching tldraw-style behavior, and the handle ids persist across reload and live sync.

Changed

• components/Canvas.tsx — onConnect keeps sourceHandle/targetHandle; edge load, persist, getBoard, and receiveEdges all carry the handle ids.
• lib/canvas-store.ts — StoredEdge gains optional sourceHandle / targetHandle fields.