[codex] reduce runtime fetch noise

[lcmadeira]

What changed

• Treat Google News as a no-CORS host and avoid direct browser fetches that fail on GitHub Pages.
• Use stale cached news when RSS refresh fails.
• Stop retrying definitive 400/404 responses through public proxies.
• Add REE mix fallback from today to yesterday.
• Disable broken ECB Euribor live fetches by default while keeping the EMMI fallback visible.
• Limit known fragile REN/OpenSky live calls to local development and reduce noisy Stooq proxy attempts.

Why

The dashboard was loading usable fallback data but the browser console was flooded with CORS, proxy, 400, and 404 errors from endpoints that either do not allow browser access or currently reject the requested URL.

Validation

• git diff --check
• node node_modules/eslint/bin/eslint.js index.html (0 errors; existing warnings remain)
• node --check src/fetch.js
• node --check src/cards/news.js
• node --check src/cards/elec.js
• Local dev server responded with 200 OK for /, /src/fetch.js, and /src/cards/news.js.