style(backend): lowercase error messages across handlers

Normalize error message casing to lowercase for consistency across auth, bookings, services, customer relationship, and profile handlers.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/auth/local.go

@@ -14,7 +14,6 @@ import (
 	"fmt"
 	"log"
 	"net/http"
-	"net/mail"
 	"regexp"
 	"strings"
 	"sync"
@@ -136,9 +135,8 @@ func RegisterHandler(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Validate email format
-	_, err := mail.ParseAddress(req.Email)
-	if err != nil {
-		http.Error(w, "invalid email format", http.StatusBadRequest)
+	if err := validators.ValidateEmail(req.Email); err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
 		return
 	}