Add upload progress bar with SSE extraction status

ISO uploads now show a progress bar during file transfer (via XHR
upload.onprogress) and real-time extraction status (via SSE events
through the existing Hub). Falls back to plain form POST if JS is
disabled.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/web/static/app.css

@@ -125,3 +125,24 @@ main { padding: 1.5rem; max-width: 1200px; margin: 0 auto; }
 .inline { display: inline; }
 h2 { margin-bottom: 1rem; }
 h3 { margin: 1.5rem 0 0.75rem; color: var(--text-muted); font-size: 0.95rem; }
+
+.upload-progress { max-width: 400px; }
+.progress-bar-track {
+  width: 100%;
+  height: 8px;
+  background: var(--bg);
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  margin: 0.75rem 0;
+  overflow: hidden;
+}
+.progress-bar-fill {
+  height: 100%;
+  width: 0%;
+  background: var(--accent);
+  border-radius: 4px;
+  transition: width 0.3s ease;
+}
+.progress-bar-fill.complete { background: var(--green); }
+.progress-text { font-size: 0.85rem; color: var(--text); margin-bottom: 0.25rem; }
+.progress-detail { font-size: 0.8rem; color: var(--text-muted); }