TASK-13: Add sidebar project list sorted by distance

[z4ab]

Closes #16

What

Adds a scrollable project list to the sidebar showing nearby projects sorted by distance from the map center.

Backend Changes

• backend/queries.py — spatial_search now includes distance_km (rounded to 2 decimals) in each result row
• backend/models.py — Added distance_km: float | None to ProjectDetail

Frontend Changes

• App.tsx — Lifts map center/zoom state up from MapView, passes to Sidebar
• MapView.tsx — Added onMapMove callback prop; reports position on init and on every moveend
• Sidebar.tsx — Complete rewrite:
  • Project list section fetches projects near current map center
  • Sorted by distance (closest first)
  • Each item shows: status dot, address, distance, type, value
  • Loading skeleton while fetching
  • Empty state message when no projects found
  • Compact legend at bottom
• types.ts — Added distance_km to ProjectProperties

Verification

• ruff check . — passes
• ruff format --check . — passes