feat(web): block-playout troubleshooting + sequential schedule validator screens

Two new System-area screens over the new endpoints:

- /app/troubleshooting/blocks: pick a Block playout, browse its blocks
  (grouped, name filter), drill into paged block history, click a row to
  decode its details.
- /app/troubleshooting/yaml: paste YAML, toggle import schema, validate;
  shows messages + JSON conversion.

Adds api/playoutHistory client + validateSequentialSchedule; regenerated
generated/v1.d.ts. Async loads follow the LogsScreen seq-guard pattern
(no synchronous setState inside effects).

Refs #145 #158

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/src/shell.css

@@ -3434,6 +3434,51 @@
   word-break: break-word;
 }
 
+.ctv-textarea {
+  width: 100%;
+  padding: 10px 12px;
+  border-radius: var(--radius-sm);
+  background: var(--ctv-surface-2);
+  border: 1px solid var(--border-hairline);
+  color: var(--text-primary);
+  font: var(--text-xs) / 1.5 var(--font-mono);
+}
+
+.ctv-textarea:focus {
+  outline: none;
+  border-color: var(--status-accent, var(--ctv-accent, #4c8bf5));
+}
+
+.ctv-validation-messages {
+  margin: 0;
+  padding-left: 20px;
+  display: flex;
+  flex-direction: column;
+  gap: var(--space-3, 6px);
+  font: var(--text-xs) / 1.5 var(--font-mono);
+}
+
+.ctv-detail-grid {
+  display: grid;
+  grid-template-columns: minmax(140px, max-content) 1fr;
+  gap: var(--space-4, 8px) var(--space-7, 16px);
+  margin: 0;
+}
+
+.ctv-detail-grid dt {
+  color: var(--text-disabled);
+  font-size: var(--text-2xs, 11px);
+  font-weight: var(--weight-medium, 500);
+  letter-spacing: var(--tracking-caps, 0.06em);
+  text-transform: uppercase;
+  align-self: center;
+}
+
+.ctv-detail-grid dd {
+  margin: 0;
+  color: var(--text-primary);
+}
+
 .ctv-settings-flush-row {
   display: flex;
   align-items: center;