Add comprehensive i18n localization support

- Add go-i18n package for internationalization
- Create i18n middleware to extract Accept-Language header
- Add translation files for en, es, fr, de, pt languages
- Localize all handler error messages and responses
- Add language context to all API handlers

Supported languages: English, Spanish, French, German, Portuguese

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/handlers/user_handler.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/gin-gonic/gin"
 
+	"github.com/treytartt/casera-api/internal/i18n"
 	"github.com/treytartt/casera-api/internal/middleware"
 	"github.com/treytartt/casera-api/internal/models"
 	"github.com/treytartt/casera-api/internal/services"
@@ -46,7 +47,7 @@ func (h *UserHandler) GetUser(c *gin.Context) {
 
 	userID, err := strconv.ParseUint(c.Param("id"), 10, 32)
 	if err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid user ID"})
+		c.JSON(http.StatusBadRequest, gin.H{"error": i18n.LocalizedMessage(c, "error.invalid_user_id")})
 		return
 	}
 
@@ -54,7 +55,7 @@ func (h *UserHandler) GetUser(c *gin.Context) {
 	targetUser, err := h.userService.GetUserIfSharedResidence(uint(userID), user.ID)
 	if err != nil {
 		if err == services.ErrUserNotFound {
-			c.JSON(http.StatusNotFound, gin.H{"error": "User not found"})
+			c.JSON(http.StatusNotFound, gin.H{"error": i18n.LocalizedMessage(c, "error.user_not_found")})
 			return
 		}
 		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})