Shapefile import: UI support (parent #228, ticket B)

[spydmobile]

Parent: #228
Depends on: #269 (shapefile server-side validation + ingestion)

Add shapefile as a perimeter upload option in the UI, wired to the unified server-side import endpoint. Server-side validation is delivered separately in #269 (which depends on the #267 refactor).

Scope

• Add "Shapefile" to the perimeter upload format selector in the wizard / import flow
• Accept either a zipped shapefile (.zip containing .shp + sidecars) or a direct multi-file selection of .shp + sidecars
• Upload to the unified server endpoint (introduced in Refactor: unify server-side perimeter validation (GeoJSON + KML) #267, extended for shapefile in Shapefile import: server-side validation + ingestion #269)
• Surface server validation errors inline (same UX pattern as the unified endpoint provides for GeoJSON/KML)
• Update inline help / tooltips to mention shapefile support
• Update relevant docs (user-facing import instructions)

Out of scope

• Server-side validation or parsing (delivered in Shapefile import: server-side validation + ingestion #269)
• Any change to the underlying GDAL ingestion logic

Acceptance

• A user can drag-and-drop a zipped shapefile and have the perimeter import succeed
• Validation errors from the server are displayed clearly inline (not buried in a generic error)
• Existing GeoJSON / KML upload paths continue to work unchanged
• Frontend tests cover the new format option in the selector and the error-display path