feat: prerelease workflow with major version support and version propagation

- Add docker-dev.yml: prerelease CI for dev branch with minor/major bump
  inputs; auto-continues in-flight major line via existing pre tags;
  publishes floating major-pre Docker tag (e.g. 2-pre)
- Rewrite docker.yml version-bump: tag-based versioning, manual bump
  inputs (auto/patch/minor/major), major guarded by confirm_major=MAJOR,
  auto-finalizes in-flight prereleases; publishes floating major tag (e.g. 2)
- Inject APP_VERSION build-arg through Dockerfile so the running container
  knows its real version instead of reading package.json
- Server reads APP_VERSION env in authService/adminService; exposes
  is_prerelease in app config and update-check response; prerelease builds
  compare against GitHub prerelease releases rather than latest stable
- Client stores isPrerelease from config; navbar shows amber version badge
  on prerelease builds (left of dark-mode toggle); GitHubPanel filters out
  prerelease releases unless the running build is itself a prerelease

client/src/pages/AdminPage.tsx

@@ -55,6 +55,7 @@ interface UpdateInfo {
   current: string
   release_url?: string
   is_docker?: boolean
+  is_prerelease?: boolean
 }
 
 const ADMIN_EVENT_LABEL_KEYS: Record<string, string> = {
@@ -1370,7 +1371,7 @@ export default function AdminPage(): React.ReactElement {
 
           {activeTab === 'mcp-tokens' && <AdminMcpTokensPanel />}
 
-          {activeTab === 'github' && <GitHubPanel />}
+          {activeTab === 'github' && <GitHubPanel isPrerelease={updateInfo?.is_prerelease ?? false} />}
 
           {activeTab === 'dev-notifications' && <DevNotificationsPanel />}
         </div>