feat: add trusted mirror update source

Version checks and downloads previously depended on GitHub Release. About & Updates now offers GitHub or a validated mirror source, keeps the selector beneath the startup-check switch, and disables it when startup checks are off.

The website resolves platform downloads from a bounded OSS release index with a GitHub fallback. Tagged releases publish and verify immutable OSS assets through OIDC before switching the latest-version index; deployment requires the configured Alibaba Cloud environment variables and role.

Release note: “关于与更新”新增 GitHub 与镜像节点选择，启动检查、手动检查和下载页使用同一可信来源；官网下载也可按系统、架构和安装包类型直接选择。

sites/styles.css

@@ -1258,6 +1258,39 @@ p {
   font-size: 0.75rem;
 }
 
+.download-options {
+  display: grid;
+  grid-column: 1 / -1;
+  grid-template-columns: repeat(2, minmax(0, 1fr));
+  gap: var(--space-3);
+}
+
+.download-options label {
+  display: grid;
+  gap: var(--space-2);
+  color: var(--text-muted);
+  font-size: var(--font-caption);
+  font-weight: 680;
+}
+
+.download-options select {
+  width: 100%;
+  min-height: 40px;
+  padding: 0 var(--space-3);
+  border: 1px solid var(--border-control);
+  border-radius: var(--radius-control);
+  background: var(--surface-subtle);
+  color: var(--text-primary);
+  font: inherit;
+  font-size: 0.75rem;
+}
+
+.download-options select:focus-visible {
+  border-color: var(--accent);
+  outline: 2px solid color-mix(in srgb, var(--accent) 28%, transparent);
+  outline-offset: 2px;
+}
+
 .platform-icon {
   display: grid;
   width: 48px;
@@ -1278,14 +1311,66 @@ p {
   stroke-width: 1.3;
 }
 
-.download-card:nth-child(3) .platform-icon svg {
-  fill: none;
+.platform-icon img {
+  width: 28px;
+  height: 28px;
+  object-fit: contain;
+}
+
+[data-theme="dark"] .platform-icon img {
+  filter: invert(1);
 }
 
 .button--download {
   grid-column: 1 / -1;
-  width: 100%;
+  width: fit-content;
+  min-height: 40px;
+  justify-content: flex-start;
+  justify-self: start;
+  padding: var(--space-2) 0;
   margin-top: var(--space-2);
+  border: 0;
+  background: transparent;
+  color: var(--accent);
+  overflow-wrap: anywhere;
+  text-align: left;
+  text-decoration: underline;
+  text-decoration-color: transparent;
+  text-underline-offset: 4px;
+}
+
+.button--download:not(.is-disabled):hover {
+  color: var(--accent-hover);
+  text-decoration-color: currentColor;
+  transform: translateX(2px);
+}
+
+.download-meta {
+  grid-column: 1 / -1;
+  min-height: 18px;
+  text-align: left;
+}
+
+.download-release-status {
+  padding: var(--space-3) var(--space-4);
+  margin-top: var(--space-4);
+  border: 1px solid var(--border-default);
+  border-radius: var(--radius-control);
+  background: var(--surface-subtle);
+  color: var(--text-secondary);
+  font-size: 0.75rem;
+  text-align: center;
+}
+
+.download-release-status.is-ready {
+  border-color: color-mix(in srgb, var(--success) 34%, var(--border-default));
+  background: var(--success-subtle);
+  color: var(--success);
+}
+
+.download-release-status.is-fallback {
+  border-color: color-mix(in srgb, var(--warning) 34%, var(--border-default));
+  color: var(--text-secondary);
 }
 
 .domestic-support {
@@ -1601,7 +1686,7 @@ p {
   }
 
   .download-card {
-    grid-template-columns: auto 1fr auto;
+    grid-template-columns: auto 1fr;
     align-items: center;
   }
 
@@ -1610,9 +1695,8 @@ p {
   }
 
   .button--download {
-    grid-column: auto;
-    width: auto;
-    margin-top: 0;
+    grid-column: 1 / -1;
+    width: fit-content;
   }
 
   .assistant-grid {
@@ -1802,9 +1886,13 @@ p {
     grid-template-columns: auto 1fr;
   }
 
+  .download-options {
+    grid-template-columns: 1fr;
+  }
+
   .button--download {
     grid-column: 1 / -1;
-    width: 100%;
+    width: fit-content;
   }
 
   .domestic-support {