feat: show upload size limit and header extraction info in web UI

Explain to users that BAM headers are extracted in the browser and
CRAM/FASTA uploads are limited to 4MB. This addresses the UX concern
of users not knowing limits before attempting large uploads.

Author: nh13

src/web/static/css/styles.css

@@ -721,6 +721,13 @@ button:disabled {
     white-space: pre;
 }
 
+.upload-hint {
+    color: var(--text-muted);
+    font-size: 0.8rem;
+    line-height: 1.5;
+    margin-bottom: 0.75rem;
+}
+
 .extraction-status {
     color: var(--accent);
     font-size: 0.85rem;

src/web/templates/index.html

@@ -143,7 +143,11 @@ <h2>Input & Configuration</h2>
             <div class="tab-content" id="tab-binary">
                 <div class="card-body">
                     <div class="tab-description">
-                        Upload BAM, CRAM, or FASTA files to automatically extract sequence information from headers. These files are uploaded and processed server-side.
+                        Upload BAM, CRAM, or FASTA files to automatically extract sequence information from headers.
+                    </div>
+                    <div class="upload-hint">
+                        <strong>BAM:</strong> Headers are extracted in your browser &mdash; large files are fine.<br>
+                        <strong>CRAM/FASTA:</strong> Upload limit is 4MB (headers only).
                     </div>
                     <div style="display: flex; gap: 1rem; margin-bottom: 1rem; flex-wrap: wrap;">
                         <label for="binary-file-upload" class="demo-button" style="cursor: pointer;">📄 Upload</label>