Add English and French i18n support

[haim1120]

Summary

• Add next-intl routing with /en and /fr locale paths plus redirects from the default English URLs.
• Move visible UI copy into English and French message files.
• Add a navbar language switcher and locale-aware links across the app.

Verification

• git diff --check
• Parsed both src/messages/en.json and src/messages/fr.json with Node
• npm install currently fails in the standalone checkout because @sorosave/sdk is declared as workspace:* but the workspace package is not included in this repo. To verify the frontend changes anyway, I ran npm install --ignore-scripts and npm run build in a temporary copy with @sorosave/sdk pointed at a local SDK stub. The build passed successfully.

Fixes #16

Transparency note: AI-assisted with Codex; I reviewed the diff and verified it locally before submission.