Component shopping cart and BOM export with supplier links

## Summary

Users have no way to get from a finished circuit design to actually buying the parts. A shopping cart / bill of materials panel would close that gap.

## Proposed features

- **BOM panel** — list every placed component with quantity, value, and package
- **Supplier links** — auto-generate deep links to DigiKey, Mouser, and LCSC for each part
- **Export** — download the BOM as CSV or JSON for use in other tools
- **Cart view** — aggregate quantities across multiple instances of the same part

## Implementation notes

- Component metadata needed for supplier lookup is already partially present in `component.json` (id, name, power)
- Could add an optional `supplier` field to `component.json` for explicit part numbers
- BOM could be a new pane alongside the existing schematic / code panes (`src/panes/`)

Start by inspecting the existing panes in src/panes/ and the fields currently available in component.json. Trace how placed components are represented so the BOM can aggregate instances and expose supplier metadata. Done means a BOM pane lists quantities, values, and packages, provides DigiKey, Mouser, and LCSC links, and exports CSV or JSON.

Written by the indexing model from the issue text.