fix(view-browser): elevate header so Recent runs dropdown overlays main content (#105)

The Recent-runs dropdown menu was being clipped from below by the
highlighted scan-path `<code>` block in `<main>`. The dropdown's
`z-index: 10` only competed inside its own stacking context — the
parent `.recent-scans-menu` was `position: relative` but `<header>`
had no positioning, so source order made `<main>` paint above the
header by default.

Promote `<header>` to `position: relative; z-index: 100` so it owns a
stacking context above `<main>`. Bump `.recent-scans-list` z-index
from 10 → 1000 inside that context (defensive overkill for any future
header-internal layering needs).

Affects only the browser interface (`argus view --interface=browser`).
Templates unchanged. All 147 browser-viewer tests pass.

Co-authored-by: eFAILution <eFAILution@users.noreply.github.com>

Author: eFAILution

argus/viewers/browser/static/argus.css

@@ -168,6 +168,12 @@ header {
   margin-bottom: 1.5rem;
   flex-wrap: wrap; gap: 0.5rem 1rem;
   padding-top: 1.25rem; padding-bottom: 1.25rem;
+  /* Promote the header into its own stacking context above <main> so
+   * absolutely-positioned descendants (the Recent-runs dropdown) can
+   * overlay the highlighted scan-path block and any other downstream
+   * content. Without this, source order makes <main> paint on top. */
+  position: relative;
+  z-index: 100;
 }
 header .brand {
   display: flex; align-items: center; gap: 0.6rem;
@@ -265,7 +271,10 @@ header nav a[aria-current="page"] {
   margin: 0; list-style: none;
   min-width: 20rem;
   max-height: 24rem; overflow-y: auto;
-  z-index: 10;
+  /* High z-index inside the header's stacking context (header itself
+   * is z-index: 100). Combined, this guarantees the open dropdown
+   * paints above any <main> content beneath. */
+  z-index: 1000;
   box-shadow: 0 6px 24px rgba(0, 0, 0, 0.5);
 }
 .recent-scans-list li { margin: 0; }