fix: respect imperial units in wall panel

[MateoSaettone]

Summary

• add shared linear measurement helpers for meter/imperial formatting and conversion
• reuse the shared formatter across wall, fence, site edge, item placement, and floorplan measurement labels
• refactor the existing imperial-aware MetricControl to use the shared conversion helpers instead of its own feet multiplier
• display/edit wall metadata dimensions in ft when the viewer unit is imperial while keeping scene values in meters

After the fix, imperial mode keeps the selected wall metadata panel in ft while scene values continue to persist internally in meters.

Closes #322

Test Plan

• bun test packages/editor/src/lib/measurements.test.ts
• bunx biome check packages/editor/src/lib/measurements.ts packages/editor/src/lib/measurements.test.ts packages/editor/src/index.tsx packages/editor/src/components/editor/wall-measurement-label.tsx packages/editor/src/components/editor/site-edge-labels.tsx packages/editor/src/components/editor/floorplan-panel.tsx packages/editor/src/components/tools/item/use-placement-coordinator.tsx packages/editor/src/components/ui/controls/metric-control.tsx packages/nodes/src/wall/tool.tsx packages/nodes/src/wall/panel.tsx packages/nodes/src/fence/tool.tsx
• bun run --cwd packages/editor check-types
• bun run check-types

Notes

• Editable numeric controls continue to use decimal feet (for example, 10.50 ft); passive measurement labels continue to use feet/inches (for example, 10'6").
• bun run --cwd packages/nodes build currently fails on an unrelated existing type error in packages/nodes/src/site/renderer.tsx.