[style](trx-frontend-http): sticky APRS bar header on scroll

Make the APRS/Clear header row position:sticky so it stays visible
when the frame list is scrolled. Adds a subtle backdrop-blur background
and a divider border so frames slide cleanly beneath it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/style.css

@@ -646,6 +646,12 @@ small { color: var(--text-muted); }
   gap: 0;
 }
 .aprs-bar-header {
+  position: sticky;
+  top: 0;
+  z-index: 1;
+  background: color-mix(in srgb, var(--card-bg) 80%, transparent);
+  backdrop-filter: blur(8px);
+  -webkit-backdrop-filter: blur(8px);
   display: flex;
   align-items: center;
   justify-content: space-between;
@@ -655,8 +661,10 @@ small { color: var(--text-muted); }
   letter-spacing: 0.12em;
   color: var(--accent-green);
   text-transform: uppercase;
-  margin-bottom: 0.12rem;
-  opacity: 0.85;
+  padding-bottom: 0.1rem;
+  margin-bottom: 0.1rem;
+  border-bottom: 1px solid color-mix(in srgb, var(--border-light) 50%, transparent);
+  opacity: 0.95;
 }
 .aprs-bar-clear {
   background: none;