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/postgres.go

@@ -204,6 +204,18 @@ func (s *PostgresStore) GetCollection(id string) (Collection, error) {
 	return c, nil
 }
 
+func (s *PostgresStore) DeleteCollection(id string) error {
+	res, err := s.db.Exec(`DELETE FROM collections WHERE id = $1`, id)
+	if err != nil {
+		return err
+	}
+	n, _ := res.RowsAffected()
+	if n == 0 {
+		return ErrNotFound
+	}
+	return nil
+}
+
 func (s *PostgresStore) SaveFeature(f Feature) {
 	geom, _ := json.Marshal(f.Geometry)
 	props, _ := json.Marshal(f.Properties)