Move competition back button onto the page with a real icon

The back button moves out of the topbar to the top-left of the page
above the competition name, and now renders an inline SVG arrow
(iconArrowLeft) instead of the "←" character.

web/style.css

@@ -403,6 +403,16 @@ tbody tr.transferred { background: #f3f6fb; }
    unused descender space, so its glyphs sit above the flex centre line. */
 .ws-indicator .connection-status { position: relative; top: 1px; }
 
+.btn-back {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.35rem;
+  margin-bottom: 0.75rem;
+  padding-left: 0.3rem;
+}
+.icon { display: inline-flex; }
+.icon svg { display: block; }
+
 textarea { resize: vertical; min-height: 60px; }
 
 @media (max-width: 700px) {