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/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 {