fix: block renaming main project variant; dynamic page titles

- Add ErrCannotRenameMainVariant; ProjectService.Update now returns
  this error if the caller tries to change the Variant of a main
  project (empty Variant) — ensures there is always exactly one main
- Handle ErrCannotRenameMainVariant in PUT /api/projects/:uuid with 400
- Set document.title dynamically from breadcrumb data:
  - Configurator: "CODE / variant / Config name — QuoteForge"
  - Project detail: "CODE / variant — QuoteForge"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/templates/index.html

@@ -477,6 +477,7 @@ function updateConfigBreadcrumbs() {
     configEl.textContent = truncateBreadcrumbSpecName(fullConfigName);
     configEl.title = fullConfigName;
     versionEl.textContent = 'main';
+    document.title = code + ' / ' + variant + ' / ' + fullConfigName + ' — QuoteForge';
     const configNameLinkEl = document.getElementById('breadcrumb-config-name-link');
     if (configNameLinkEl && configUUID) {
         configNameLinkEl.href = '/configs/' + configUUID + '/revisions';