fix(navigation): unclip dropdowns and flip toward the side with more space

- Move <header> overflow:hidden into body.fullscreen scope and drop the
  implicit-vertical-clip overflow-x:auto from .navbar-nav so dropdown
  menus can escape the navbar.
- Drive top-level dropdowns through bootstrap.Dropdown (popperConfig
  strategy:'fixed'), and add a chooseDirection() helper that toggles
  .dropup/.dropdown on the .nav-item based on space above vs below
  before each show. Split the navigation.css rules to position the menu
  with top:100% or bottom:100% accordingly.
- Mark the dropdown toggle with data-bs-toggle="dropdown" in the
  template; cover that with a Jinja-rendered unit test and add Cypress
  specs for the header (opens downward) and footer (flips to .dropup)
  cases.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/templates/moduls/navigation.html.j2

@@ -85,7 +85,7 @@
                 {% else %}        
                 <!-- Dropdown Menu -->
                     <li class="nav-item dropdown">
-                        <a class="nav-link dropdown-toggle btn btn-light" id="navbarDropdown{{ loop.index }}" role="button" data-bs-display="dynamic" aria-expanded="false">
+                        <a class="nav-link dropdown-toggle btn btn-light" id="navbarDropdown{{ loop.index }}" role="button" data-bs-toggle="dropdown" data-bs-display="dynamic" aria-expanded="false">
                             {% if item.icon is defined and item.icon.class is defined %}
                                 {{ render_icon_and_name(item) }}
                             {% else %}