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/default.css

@@ -111,17 +111,13 @@ div#navbarNavfooter li.nav-item {
   margin-right: 6px;
 }
 
-/* Prevent nav items from wrapping to a second line */
+/* Prevent nav items from wrapping to a second line.
+   overflow is intentionally NOT set here — overflow-x:auto would
+   implicitly clip overflow-y too and hide dropdown menus that open
+   below the navbar. */
 div#navbarNavheader .navbar-nav,
 div#navbarNavfooter .navbar-nav {
   flex-wrap: nowrap;
-  overflow-x: auto;
-  scrollbar-width: none; /* Firefox */
-}
-
-div#navbarNavheader .navbar-nav::-webkit-scrollbar,
-div#navbarNavfooter .navbar-nav::-webkit-scrollbar {
-  display: none; /* Chrome/Safari */
 }
 
 main, footer, header, nav {
@@ -197,7 +193,6 @@ iframe{
 /* 1. Make sure headers and footers can collapse */
 header,
 footer {
-  overflow: hidden;
   /* choose a max-height that’s >= your tallest header/footer */
   max-height: 200px;
   padding: 1rem;
@@ -206,9 +201,11 @@ footer {
     padding var(--anim-duration) ease-in-out;
 }
 
-/* 2. In fullscreen mode, collapse them */
+/* 2. In fullscreen mode, collapse them. overflow: hidden is scoped here
+   so dropdown menus can escape the header in normal mode. */
 body.fullscreen header,
 body.fullscreen footer {
+  overflow: hidden;
   max-height: 0;
   padding-top: 0;
   padding-bottom: 0;