fix: Resolve mixed language content and improve i18n implementation (#54)

[Harikrishnan1367709]

Problem:

• Mixed French/English content when switching from French to English
• Incomplete French translations with typos
• Unprofessional appearance due to inconsistent language display

Solution:

• ✅ Fixed hardcoded English strings in components (Stats, Pricing, FirstFeatures, Testimonials)
• ✅ Completed French translations and corrected typos
• ✅ Enhanced language switching mechanism with proper page reload
• ✅ Added conditional header language selector (hidden when French is selected)
• ✅ Resolved build errors (duplicate keys, translation namespaces)

Technical Changes:

• Migrated hardcoded strings to next-intl translations
• Updated all locale files (en.json, fr.json, es.json, zh-Hans.json)
• Fixed React key uniqueness in stats component
• Corrected translation namespace in pricing component
• Improved language switching with window.location.href for full page reload

Result:

• Clean language switching with consistent content
• Professional appearance across all languages
• Resolved build errors and console warnings
• Maintained developer-focused English experience while supporting internationalization

Testing:

• ✅ Language switching works correctly
• ✅ No mixed language content
• ✅ All components properly translated
• ✅ Build errors resolved

fix #54