feat: add onboarding and French translations

[chadi-shwada]

Summary

• Add a guided onboarding flow inside the workspace to help users connect a model provider and start using Sinew.
• Add a Settings action to replay the onboarding.
• Add English/French language switching for the main app UI, including settings, chat empty states, conversations, search, welcome, tool labels, default tool descriptions, and the Plan mode prompt display.

Tests

• git diff --check
• npm.cmd run build