Add timezone backend support (handlers, utilities, user context)

- Add FormatInTimezone and FormatTimestamptzInTimezone helpers
  in templates/utils.go for timezone-aware time display
- Add Timezone field to templates.User struct
- Pass user timezone from DB to template context in helpers.go
- Add timezone update handling in auth.go UpdateProfile with
  validation via time.LoadLocation
- Add UpdateTimezoneSettings handler in system_settings.go for
  admin system-wide default timezone using UpdateSystemSetting

internal/handlers/auth.go

@@ -87,6 +87,7 @@ type UpdateProfileRequest struct {
 	FirstName string `json:"first_name,omitempty" validate:"omitempty,max=100"`
 	LastName  string `json:"last_name,omitempty" validate:"omitempty,max=100"`
 	Theme     string `json:"theme,omitempty" validate:"omitempty"`
+	Timezone  string `json:"timezone,omitempty" validate:"omitempty"`
 }
 
 type AdminUpdateUserRequest struct {
@@ -564,6 +565,22 @@ func (h *AuthHandler) UpdateProfile(c *echo.Context) error {
 		}
 	}
 
+	// Update timezone
+	if req.Timezone != "" {
+		if _, err := time.LoadLocation(req.Timezone); err != nil {
+			return c.JSON(http.StatusBadRequest, map[string]string{
+				"error": "Invalid timezone",
+			})
+		}
+		err := h.db.UpdateUserTimezone(ctx, database.UpdateUserTimezoneParams{
+			ID:       pgtype.UUID{Bytes: userUUID, Valid: true},
+			Timezone: pgtype.Text{String: req.Timezone, Valid: true},
+		})
+		if err != nil {
+			return err
+		}
+	}
+
 	// Update email (if provided)
 	if req.Email != "" {
 		existingUser, err := h.db.GetUserByEmail(c.Request().Context(), req.Email)

internal/handlers/system_settings.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"net/http"
 	"strconv"
+	"time"
 
 	"github.com/jackc/pgx/v5"
 	"github.com/labstack/echo/v5"
@@ -31,6 +32,28 @@ type ScanSettingsResponse struct {
 	Message                 string `json:"message,omitempty"`
 }
 
+type UpdateTimezoneSettingsRequest struct {
+	DefaultTimezone string `json:"default_timezone" validate:"required"`
+}
+
+func (h *SystemSettingsHandler) UpdateTimezoneSettings(c *echo.Context) error {
+	var req UpdateTimezoneSettingsRequest
+	if err := c.Bind(&req); err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "Invalid request"})
+	}
+	if _, err := time.LoadLocation(req.DefaultTimezone); err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "Invalid timezone"})
+	}
+	err := h.db.UpdateSystemSetting(c.Request().Context(), database.UpdateSystemSettingParams{
+		SettingKey:   "default_timezone",
+		SettingValue: req.DefaultTimezone,
+	})
+	if err != nil {
+		return err
+	}
+	return c.JSON(http.StatusOK, map[string]string{"message": "Timezone updated"})
+}
+
 func (h *SystemSettingsHandler) UpdateScanSettings(c *echo.Context) error {
 	var req UpdateScanSettingsRequest
 	if err := c.Bind(&req); err != nil {

internal/router/helpers.go

@@ -35,6 +35,11 @@ func getTemplateUserWithTheme(c *echo.Context, cfg *Config) (templates.User, err
 		userTheme = userDB.Theme.String
 	}
 
+	userTimezone := "UTC"
+	if userDB.Timezone.Valid {
+		userTimezone = userDB.Timezone.String
+	}
+
 	// Extract JWT token for WebSocket authentication
 	token := ""
 	if cookie, err := c.Cookie("token"); err == nil {
@@ -48,6 +53,7 @@ func getTemplateUserWithTheme(c *echo.Context, cfg *Config) (templates.User, err
 		Role:     userRole,
 		Theme:    userTheme,
 		Token:    token,
+		Timezone: userTimezone,
 	}, nil
 }