feat: add in-app feedback dialog

[benvinegar]

Summary

• add an F2/menu-triggered feedback dialog with description and optional email fields
• submit feedback to Modem ingest using a public project key
• cover payload construction, submission failures, and TUI interactions

Before merge

• replace the placeholder Modem public key in src/core/feedback.ts with the production Hunk project key

Testing

• bun run typecheck
• bun run lint
• bun test
• bun run test:integration