Fix unreachable index page and duplicate homepage entry in navigation

buildUrl() hardcoded 'index' as the homepage, so the menu link for
index.md pointed at /nl. With a different default_page that root URL
served another page, making index.md unreachable.

- Add getEffectiveDefaultPage(): resolves 'auto' to the detected page
  and caches the result
- buildUrl() now omits the page segment only for the effective default
  page instead of the literal string 'index'
- Route getPage(), generateBreadcrumb(), getContentType() and the
  render() template data (default_page, homepage, is_homepage,
  home_active_class, current_page, lang switch URLs) through it
- getHomepageTitle() returns t('home') so the home button no longer
  duplicates a menu item label
- Drop the now-redundant default_page skip in renderMenu() so every
  page stays reachable from the menu
- getAllContentDirs() also skips dot-directories (.git) in the move
  dropdown

public/admin.php

@@ -1675,8 +1675,9 @@ function getAllContentDirs(string $contentDir, string $realContentDir): array
         if (strpos($realPath, $realContentDir) !== 0) continue;
         $relative = substr($realPath, strlen($realContentDir) + 1);
         if ($relative === false || $relative === '') continue;
-        // Skip hidden directories (starting with -)
-        if (strpos(basename($relative), '-') === 0) continue;
+        // Skip hidden directories (starting with - or .)
+        $base = basename($relative);
+        if ($base[0] === '-' || $base[0] === '.') continue;
         $dirs[] = $relative;
     }