Add metro area codes and switch airport data to lxndrblz/Airports

[rhinot]

Summary

• Rewrites scripts/build-airport-map.mjs to fetch airports.csv + citycodes.csv from lxndrblz/Airports at build time — no local file or airport-timezone devDependency needed; OurAirports approach preserved in comments as backup
• Regenerates netlify/airport-map-data.json: 9,803 airports + 27 metro codes + 5 derived = 9,835 total entries
• Adds metro area code support (NYC, LON, TYO, PAR, CHI, etc.) — overrides in common.js trimmed to only the 4 not covered by the new data (TCI collision, QDF/QHO/QMI absent from dataset)
• Updates homepage to document timezone abbreviation, airport code, and metro area code inputs
• Fixes vertical centering on homepage (min-height: 100vh)
• Cleans up .gitignore (adds .claude/, deno.lock, airports.csv)

Test plan

• npm run build:airports fetches and regenerates airport-map-data.json successfully
• Airport codes resolve correctly: SFO, JFK, LHR
• Metro codes resolve correctly: NYC, LON, TYO, PAR, CHI
• Overridden codes work: TCI (Tenerife), QDF (Dallas), QHO (Houston), QMI (Miami)
• Homepage renders centered with updated copy

[netlify]

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