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/static/css/navigation.css

@@ -1,9 +1,19 @@
-/* Top-level dropdown menu */
-.nav-item .dropdown-menu {
-    position: absolute; /* Important for positioning */
-    top: 100%; /* Default opening direction: downwards */
+/* Top-level dropdown menu — direction toggled by JS via .dropdown / .dropup */
+.nav-item.dropdown > .dropdown-menu,
+.nav-item.dropup > .dropdown-menu {
+    position: absolute;
     left: 0;
-    z-index: 1050; /* Ensures the menu appears above other elements */
+    z-index: 1050;
+}
+
+.nav-item.dropdown > .dropdown-menu {
+    top: 100%;
+    bottom: auto;
+}
+
+.nav-item.dropup > .dropdown-menu {
+    top: auto;
+    bottom: 100%;
 }
 
 /* Submenu position */