fix(admin): wire up default timezone setting in admin settings page

The admin settings timezone dropdown was incomplete: it had no
pre-selection of the current value, was missing consistent styling,
and the form submission did not persist timezone changes.

Changes:
- frontend.go: load default_timezone from system_settings into the
  systemConfig map passed to the template
- admin_settings.templ: match card styling used by the Base URL
  section; pre-select current timezone with selected?= attribute
- sidecar.go: handle default_timezone in UpdateSystemConfiguration
  by writing to system_settings table instead of system_config;
  update HTMX response to include timezone section with current value
- Add selectedAttr() helper for HTMX HTML string response

internal/router/frontend.go

@@ -932,7 +932,13 @@ func registerFrontendRoutes(cfg *Config) {
 		}
 
 		systemConfig := map[string]string{
-			"base_url": baseURL,
+			"base_url":         baseURL,
+			"default_timezone": "UTC",
+		}
+
+		defaultTimezone, err := cfg.Queries.GetSystemTimezone(c.Request().Context())
+		if err == nil && defaultTimezone != "" {
+			systemConfig["default_timezone"] = defaultTimezone
 		}
 
 		var buf bytes.Buffer