feat(maestro): port the sidebar to React

Ports catalog.ts's tree renderer to components under
src/components/sidebar/, following docs/SIDEBAR.md: two-line cards
behind a 5px activity rail, adaptive meta line (duration, detail,
program, diff stat), collapsible folders, j/k walk, inline rename and a
right-click menu.

The nav walk reads a pure model (model.ts) rather than the DOM, so the
rule that an expanded folder header is skipped and a collapsed one is
not is testable without a render. Rail motion needs @keyframes, so it
stays in sidebar.css rather than the Tailwind config.

Closes #244

web/src/components/sidebar/sidebar.css

@@ -0,0 +1,49 @@
+/* Motion for the activity rail (docs/SIDEBAR.md): a hard blink while
+   AwaitingInput, a fast flash while Error. Active and Idle are solid and
+   still — colour alone is the signal. Kept as real CSS rather than Tailwind
+   utilities because both need @keyframes of their own. */
+
+@keyframes maestro-rail-blink {
+  0%,
+  50% {
+    opacity: 1;
+  }
+  50.01%,
+  100% {
+    opacity: 0;
+  }
+}
+
+@keyframes maestro-rail-flash {
+  0%,
+  100% {
+    opacity: 1;
+  }
+  15% {
+    opacity: 0.25;
+  }
+  30% {
+    opacity: 1;
+  }
+  45% {
+    opacity: 0.25;
+  }
+  60% {
+    opacity: 1;
+  }
+}
+
+.rail-blink {
+  animation: maestro-rail-blink 1.2s steps(1) infinite;
+}
+
+.rail-flash {
+  animation: maestro-rail-flash 1.4s ease-out infinite;
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .rail-blink,
+  .rail-flash {
+    animation: none;
+  }
+}