feat: add session project management

[xiami762]

Summary

• group sessions under a stable default project and user-managed projects
• support creating and renaming projects from the session sidebar
• support creating sessions directly within a selected project
• add project creation support to both Python and TUI server implementations

Validation

• npm run test:run -- src/pages/Session/index.test.tsx
• npm run lint
• npm run build
• bun build flocks/project/project.ts flocks/server/routes/project.ts --target=bun