feat(ui): redesign onboarding and file review workflow

[mssssss123]

Summary

• Port the frontend redesign from feat(ui): redesign onboarding and file review workflow #531 onto the current main branch.
• Preserve mainline behavior for onboarding/config readiness, transactional configuration saves, attachment history, queued-message controls, and message actions.
• Add the redesigned onboarding, composer, sidebar, file review, project conversation flow, model capability controls, dark-mode polish, and native workspace picker.
• Reconnect the Web UI to the Gateway after supervised runtime restarts.

Mainline preservation

• Based directly on upstream/main at f7f92e9.
• The branch is 0 commits behind main and introduces 10 focused commits.
• All attachment-history files from fix(attachments): preserve upload history safely #545 remain unchanged.
• Transactional configuration and environment-credential behavior from fix(settings): make config saves transactional #546 is retained and covered by the existing tests.
• Placeholder-free onboarding and runtime coordination from refactor: remove bootstrap placeholder configuration #547 is retained.
• No files from main are deleted by this branch.

Validation

• UI TypeScript typecheck passed.
• UI production build passed.
• 191 mainline-protection tests passed.
• 293 redesign and integration tests passed.
• Full UI run: 1075 tests passed. The remaining stream-smoother assertions and Playwright/Vitest collection issue reproduce from main-equivalent runtime/test code and are not introduced by this branch.
• Core build passed. The existing BackgroundTaskRuntime cancellation reproduces in files identical to main.
• git diff --check passed.

Relationship to the earlier PR

Supersedes #531. The original redesign work came from @W-Kevin; this PR rebases and integrates that work while preserving newer mainline behavior.

cc @W-Kevin

The earlier PR can remain open until this replacement is merged, then be closed as superseded.