fix(ui): move theme checkmark when theme changes in Appearance menu

The checkmark in the Appearance theme menu was rendered server-side
(if user.Theme == opt.Name), so it never moved after switching themes
in the browser.

- Always render the check for every theme option, hidden by default,
  using a new themeCheckClass(name, current) helper that returns the
  hidden class unless the option is the active theme.
- Give each theme button a data-theme attribute and add
  updateThemeIndicators() to web/src/theme.ts, which reads the applied
  theme from the body class (theme-<name>) and toggles the hidden class
  on each check accordingly.
- Call updateThemeIndicators() from changeTheme (before the async save
  and on failure), initializeTheme, and loadUserTheme so the menu stays
  in sync with the applied theme.
- Add unit test for themeCheckClass (templates/utils_test.go) and
  regenerate templ output.

templates/utils_test.go

@@ -45,3 +45,12 @@ func TestGroupTunableSettingsEmpty(t *testing.T) {
 		t.Errorf("expected empty groups, got live=%d restart=%d", len(live), len(restart))
 	}
 }
+
+func TestThemeCheckClass(t *testing.T) {
+	if got := themeCheckClass("dracula", "dracula"); got != "" {
+		t.Errorf("active theme should be visible, got %q", got)
+	}
+	if got := themeCheckClass("nord", "dracula"); got != " hidden" {
+		t.Errorf("inactive theme should be hidden, got %q", got)
+	}
+}