feat: Sample Tag Viewer — read-only tag browser for AKAI disk images

Backend:
- extract.go: listTags() navigates to volume, runs akaiutil lstags
- extract.go: parseTags() parses tag name/index from lstags output
- serve.go: GET /api/disk/partitions — list partitions on an ISO
- serve.go: GET /api/disk/volumes — list volumes on a partition
- serve.go: GET /api/volume/tags — list tags on a volume

Web UI:
- Tag Viewer section with ISO path input and Browse button
- Partition selector buttons
- Volume grid with click-to-select
- Tag chips with color-coded dots (20-color palette)

Tests:
- 6 parseTags tests (basic, spaces, defaults, no tags, empty, no-type)
- 1 listTags integration test
- 4 API endpoint tests (partitions, missing-ISO, missing-params, tags)

Karpathy: read-only view first — no write operations yet (settagi, clrtagi)

web/ui/style.css

@@ -387,6 +387,130 @@ h2 {
   gap: 4px;
 }
 
+/* Tag viewer */
+.tag-controls {
+  display: flex;
+  gap: 8px;
+  flex-wrap: wrap;
+}
+.tag-controls input {
+  flex: 1;
+  min-width: 200px;
+  padding: 10px 14px;
+  border: 1px solid var(--bg3);
+  border-radius: var(--radius);
+  background: var(--bg);
+  color: var(--text);
+  font-size: 14px;
+}
+.tag-controls button {
+  padding: 10px 20px;
+  border: none;
+  border-radius: var(--radius);
+  background: var(--accent);
+  color: white;
+  font-size: 14px;
+  cursor: pointer;
+}
+.tag-controls button:hover { opacity: 0.9; }
+
+.tag-status {
+  font-size: 13px;
+  color: var(--text2);
+  margin-top: 10px;
+  margin-bottom: 8px;
+}
+
+.tag-partitions {
+  display: flex;
+  gap: 8px;
+  flex-wrap: wrap;
+  margin-bottom: 10px;
+}
+
+.tag-part-btn {
+  padding: 6px 14px;
+  border: 1px solid var(--bg3);
+  border-radius: var(--radius);
+  background: var(--bg);
+  color: var(--text);
+  font-size: 13px;
+  cursor: pointer;
+  transition: border-color 0.15s;
+}
+.tag-part-btn:hover {
+  border-color: var(--accent);
+}
+.tag-part-btn.active {
+  border-color: var(--accent);
+  background: var(--accent);
+  color: white;
+  cursor: default;
+}
+
+.tag-volumes {
+  display: grid;
+  grid-template-columns: repeat(auto-fill, minmax(180px, 1fr));
+  gap: 6px;
+  margin-bottom: 12px;
+}
+
+.vol-item {
+  padding: 8px 12px;
+  border: 1px solid var(--bg3);
+  border-radius: var(--radius);
+  background: var(--bg);
+  color: var(--text);
+  font-size: 12px;
+  cursor: pointer;
+  transition: border-color 0.15s;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+.vol-item:hover {
+  border-color: var(--accent);
+}
+.vol-item.selected {
+  border-color: var(--green);
+  background: rgba(78, 204, 163, 0.1);
+}
+
+.tag-detail {
+  margin-top: 8px;
+}
+
+.tag-detail h4 {
+  font-size: 14px;
+  color: var(--text);
+  margin: 0 0 8px;
+}
+
+.tag-list {
+  display: flex;
+  gap: 6px;
+  flex-wrap: wrap;
+}
+
+.tag-chip {
+  display: inline-flex;
+  align-items: center;
+  gap: 4px;
+  padding: 4px 10px;
+  border-radius: 12px;
+  font-size: 11px;
+  font-weight: 500;
+  background: var(--bg3);
+  color: var(--text);
+}
+
+.tag-chip .tag-dot {
+  width: 8px;
+  height: 8px;
+  border-radius: 50%;
+  flex-shrink: 0;
+}
+
 /* Sample cards */
 .sample-card {
   background: var(--bg);