Demo app, collections/features CRUD, QR codes, docs

Demo app (web/):
- Collections: select, rename, remove (x button per row), delete
- Features: add point (lon/lat validation), remove, list in selected collection
- QR codes for pk (private) and pb (public) keys
- Restore public key from private key
- 409 Conflict handled for already-registered login
- Title: Momswap Geo Backend Use-Cases Test

Backend:
- PATCH /v1/collections/{id} for rename
- DELETE /v1/collections/{id}
- Clearer lon/lat validation errors (-180..180, -90..90)

Client:
- updateCollection, deleteCollection, derivePublicKey

Docs:
- docs/frontend-development.md (demo app, local dev)
- README links to all docs

Made-with: Cursor

internal/store/interface.go

@@ -16,6 +16,7 @@ type Store interface {
 	SaveCollection(c Collection)
 	ListCollectionsByOwner(owner string) []Collection
 	GetCollection(id string) (Collection, error)
+	DeleteCollection(id string) error
 	SaveFeature(f Feature)
 	ListFeaturesByCollection(collectionID string) []Feature
 	GetFeature(featureID string) (Feature, error)