feat(web): replace CSV columns caption with downloadable example.csv link

Generates a 3-row sample CSV client-side (Blob) so users see the exact
src_login,src_pass,dst_login,dst_pass layout instead of a plain caption.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

web/src/app.css

@@ -221,6 +221,24 @@
   color: var(--fg-faint);
 }
 
+.link-btn {
+  appearance: none;
+  background: transparent;
+  border: none;
+  padding: 0;
+  cursor: pointer;
+  font: inherit;
+  font-size: 11px;
+  letter-spacing: 0.08em;
+  text-transform: uppercase;
+  color: var(--fg-dim);
+  border-bottom: 1px dashed var(--border-bright);
+}
+
+.link-btn:hover {
+  color: var(--accent-strong);
+}
+
 /* ---------- buttons ---------- */
 
 .btn {