- Add fr-FR locale with French translations (875 keys)
- Register fr-FR in locale index, LanguageToggle, and date-locale
- Add common.french key to all existing locale files
- Sync fr-FR key structure to match current en-US after cleanup
Based on #514, rebased and adapted to the current locale structure.
Closes#481
Co-authored-by: saikrishna-prathapaneni <saiprathapaneni23@gmail.com>