fix(view-browser): actually elevate header above main for dropdown overlay

PR #105 added `header { z-index: 100; }` intending to put the
Recent-runs dropdown above `<main>`. It didn't work because there's
an earlier rule:

    body > * { position: relative; z-index: 1; }

That selector has specificity (0,0,0,2) vs `header`'s (0,0,0,1), so
it overrode the bump. Both `<header>` and `<main>` ended up at
z-index 1 and source order made `<main>` paint on top — so the
dropdown was still being clipped by the highlighted scan-path
`<code>` and the dashboard cards.

Fixes by raising the header's z-index via a selector that matches
or beats `body > *`:

    body > header { z-index: 100; }

The `.recent-scans-list` z-index 1000 inside header's stacking
context already wins against the dashboard cards, so this single
rule is enough. The earlier (now ineffective) `position: relative;
z-index: 100;` block on the bare `header` selector is dropped — a
comment near the original location points readers at the new
specificity-aware rule so the cause-of-the-fix is discoverable.

All 178 browser-viewer tests still pass.

Author: eFAILution

argus/viewers/browser/static/argus.css

@@ -159,6 +159,12 @@ body::before {
   z-index: 0;
 }
 body > * { position: relative; z-index: 1; }
+/* The Recent-runs dropdown lives inside <header>; without bumping the
+ * header's stacking context above main/footer, source order makes
+ * <main> paint on top and the dropdown gets clipped by anything in
+ * the body. The selector specificity has to beat the rule above
+ * (`body > *`) — a plain `header { z-index: 100 }` does not. */
+body > header { z-index: 100; }
 
 header, main, footer { padding: 1rem 1.5rem; max-width: var(--max-width); margin: 0 auto; }
 
@@ -168,12 +174,9 @@ 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;
+  /* Stacking-context promotion lives in `body > header` above so it
+   * beats the `body > *` specificity. A plain `header { z-index: ... }`
+   * is overridden by that rule and the dropdown gets clipped by main. */
 }
 header .brand {
   display: flex; align-items: center; gap: 0.6rem;