Fallback cross-search table requests to legacy GET

[karilint]

Summary

• keep the new POST body requests as the primary cross-search table/locality path
• retry the legacy GET routes only when the backend returns the rollout-skew HTML error Cannot POST /crosssearch/...
• prevents the Occurrences list from hard-failing when the frontend is newer than the backend instance serving requests

Validation

• npm run lint:frontend
• npm run tsc:frontend
• git diff --check
• commit hook: npm run lint and npm run tsc

Follow-up for the reported Occurrences list error:

Cannot POST /crosssearch/all