feat(integrations): add review fetching and Linux keyring support

Add a cross-provider Review Center and improve credential handling.
Backend integrations fetch and normalize PRs from GitHub, GitLab,
Gitea, and Azure DevOps with improved timeouts and parsing. Credentials
now use a global lock and support secret-tool on Linux to avoid races.

- Normalize review data across providers (GitHub/GitLab/Gitea/Azure)
- Serialize credential access with OnceLock and use secret-tool on Linux
- Add frontend ReviewCenter component and related UI updates

src/lib/telemetry.ts

@@ -11,6 +11,10 @@ export function beginFrontendShutdown() {
   frontendShuttingDown = true;
 }
 
+export function resumeFrontend() {
+  frontendShuttingDown = false;
+}
+
 // Telemetry must never contain repository contents or identifying local data.
 function sanitize(message: string): string {
   return message