Add English/French i18n support to frontend

[cryptodane-leduke]

Summary

• Added next-intl routing/configuration for localized frontend routes.
• Moved app pages under frontend/src/app/[locale]/.
• Added English and French message catalogs.
• Localized navbar, wallet actions, landing page, groups pages, forms, cards, member list, progress panel, and contribution modal.
• Added a language selector to switch between English and French.

Bounty

Addresses sorosave-protocol/frontend#16.

Validation

• Parsed both message catalogs with Node JSON parsing.
• Statically checked UI source for remaining hard-coded visible English text; only brand/product names remain.
• Checked downloaded archives for path traversal: none found.
• Checked package scripts: no preinstall, install, postinstall, or prepare scripts found.
• Ran Windows Defender custom scan on the downloaded SoroSave zip and extracted folder; no new detections were reported.

Limitations

I did not run a full local build because dependency installation was intentionally avoided.

A protected dependency-resolution attempt in the standalone sorosave-protocol/frontend repo failed because @sorosave/sdk@workspace:* could not be resolved there:

ERR_PNPM_WORKSPACE_PKG_NOT_FOUND: "@sorosave/sdk@workspace:*" is in the dependencies but no package named "@sorosave/sdk" is present in the workspace

The sorosave-protocol/sorosave monorepo contains the missing sdk/ workspace package, so this PR targets the monorepo layout.

Transparency note: I used Codex as an implementation assistant, reviewed the diff, and can adjust the changes based on maintainer feedback.