feat: Underline next METAR section to be decoded

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

metar.html

@@ -36,6 +36,9 @@
         .highlight {
             background-color: yellow;
         }
+        .next-to-decode {
+            text-decoration: underline;
+        }
         #decoding-display {
             margin-top: 1em;
             padding: 1em;
@@ -876,6 +879,8 @@
                 currentMetarSections.forEach((section, index) => {
                     if (index === currentSectionIndex) {
                         parts.push(`<span class="highlight">${section.raw}</span>`);
+                    } else if (index === currentSectionIndex + 1) {
+                        parts.push(`<span class="next-to-decode">${section.raw}</span>`);
                     } else {
                         parts.push(section.raw);
                     }