feat(web): improve dashboard and session recovery

[sam2tom]

Summary

• redesign Dashboard around operational Sessions and quick actions
• add an actionable Core connection recovery entry point without pretending Web can start host services
• allow a failed latest Turn to continue in the same Session while keeping terminal Environments fail-closed

Validation

• root Node checks and tests: 67 passed
• agents-client Vitest: 214 passed
• web Vitest: 552 passed
• focused Playwright recovery checks: 2 passed
• TypeScript checks, Vite production build, and git diff --check passed

Boundaries

• no deployment
• no Core changes
• no branch or Worktree deletion