fix: preserve vinext navigation on GitHub Pages

[owjs3901]

Summary

• normalize GitHub Pages .rsc responses to vinext's expected ext/x-component MIME type
• preserve the original Response URL and body so vinext can reuse prefetched RSC payloads
• scope the workaround to the browser-only demo fetch layer

Verification

• �un run tsc --noEmit in packages/app`n- VITE_YEOLLIN_BASE_PATH=/yeollin-cms VITE_YEOLLIN_DEMO=true cargo run -p yeollin-cli -- build --skip-backend from �pps/example-app (22 routes, 0 skipped)
• live Pages .rsc response reproduced as �pplication/octet-stream; normalized response retained URL and 8,296-byte body