feat: add about page with versions, changelog, and how-to-play

- New GET /about API endpoint caches Gitea releases for 5 minutes
- AboutPanel displays client version (via Vite define), API version, collapsible changelog, and How to Play guide
- GiteaRelease and AboutResponse types added to shared package

apps/web/src/styles.css

@@ -1942,3 +1942,136 @@ body {
   height: 100%;
   transition: width 0.3s ease;
 }
+
+/* ========== About Panel ========== */
+
+.about-versions {
+  display: flex;
+  gap: 1rem;
+  flex-wrap: wrap;
+  margin-bottom: 1.5rem;
+}
+
+.about-version-card {
+  background: var(--colour-bg-secondary, #16213e);
+  border: 1px solid var(--colour-border, #0f3460);
+  border-radius: 8px;
+  padding: 0.75rem 1.25rem;
+  display: flex;
+  flex-direction: column;
+  gap: 0.25rem;
+  min-width: 160px;
+}
+
+.about-version-label {
+  color: var(--colour-text-muted);
+  font-size: 0.8rem;
+}
+
+.about-version-value {
+  color: var(--colour-accent);
+  font-size: 1.1rem;
+  font-weight: bold;
+  font-family: monospace;
+}
+
+.about-loading,
+.about-empty,
+.about-error {
+  color: var(--colour-text-muted);
+  font-size: 0.9rem;
+}
+
+.about-error {
+  color: var(--colour-danger, #e74c3c);
+}
+
+.about-releases {
+  list-style: none;
+  padding: 0;
+  margin: 0 0 1.5rem;
+  display: flex;
+  flex-direction: column;
+  gap: 0.5rem;
+}
+
+.about-release {
+  background: var(--colour-bg-secondary, #16213e);
+  border: 1px solid var(--colour-border, #0f3460);
+  border-radius: 8px;
+  overflow: hidden;
+}
+
+.about-release-header {
+  width: 100%;
+  background: none;
+  border: none;
+  cursor: pointer;
+  padding: 0.75rem 1rem;
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+  color: var(--colour-text-primary, #e0e0e0);
+  text-align: left;
+}
+
+.about-release-header:hover {
+  background: rgba(255, 255, 255, 0.05);
+}
+
+.about-release-tag {
+  font-weight: bold;
+  font-size: 0.95rem;
+  flex: 1;
+}
+
+.about-release-date {
+  color: var(--colour-text-muted);
+  font-size: 0.8rem;
+}
+
+.about-release-chevron {
+  color: var(--colour-text-muted);
+  font-size: 0.75rem;
+}
+
+.about-release-body {
+  white-space: pre-wrap;
+  word-break: break-word;
+  font-family: inherit;
+  font-size: 0.85rem;
+  color: var(--colour-text-secondary, #b0b0b0);
+  padding: 0 1rem 0.75rem;
+  margin: 0;
+  border-top: 1px solid var(--colour-border, #0f3460);
+}
+
+.about-how-to-play {
+  list-style: none;
+  padding: 0;
+  margin: 0;
+  display: flex;
+  flex-direction: column;
+  gap: 1rem;
+}
+
+.about-htp-section {
+  background: var(--colour-bg-secondary, #16213e);
+  border: 1px solid var(--colour-border, #0f3460);
+  border-radius: 8px;
+  padding: 0.75rem 1rem;
+}
+
+.about-htp-title {
+  font-size: 0.95rem;
+  font-weight: bold;
+  color: var(--colour-accent);
+  margin: 0 0 0.4rem;
+}
+
+.about-htp-body {
+  font-size: 0.85rem;
+  color: var(--colour-text-secondary, #b0b0b0);
+  margin: 0;
+  line-height: 1.5;
+}