fix: prevent comparison swap refetches - #200

Fixes #189

Summary

• Treat swapped username pairs and reordered language filters as the same comparison fetch.
• Preserve the latest presentation order in the URL and displayed data without issuing another /api/compare request.
• Reconcile in-flight completions with the latest presentation and ignore stale results or errors for a different comparison.
• Add focused regression coverage for swap, history, language, duplicate-submit, and stale-response behavior.

Validation

• 13 Vitest files and 97 tests passed
• ESLint passed
• TypeScript --noEmit passed
• Locale validation passed
• Next.js production build passed (161 pages generated)
• The exact change was manually reviewed and its swap/history behavior was personally tested

Additionally, at the time of submission of this PR:

• The referred issue is not blocked currently
• All unittests passed after changes were made