fix(web): prevent 404 on workspace switch and downgrade 404 log level

- Skip issue refetch when store is cleared during workspace switch by
  tracking which issue was already loaded (loadedIdRef pattern)
- Downgrade 404 responses from logger.error to logger.warn in ApiClient
  since resource-not-found is a normal business response, not a bug

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/shared/api/client.ts

@@ -113,7 +113,8 @@ export class ApiClient {
     if (!res.ok) {
       if (res.status === 401) this.handleUnauthorized();
       const message = await this.parseErrorMessage(res, `API error: ${res.status} ${res.statusText}`);
-      this.logger.error(`← ${res.status} ${path}`, { rid, duration: `${Date.now() - start}ms`, error: message });
+      const logLevel = res.status === 404 ? "warn" : "error";
+      this.logger[logLevel](`← ${res.status} ${path}`, { rid, duration: `${Date.now() - start}ms`, error: message });
       throw new Error(message);
     }