feat: improve settings configuration experience

[shenjiecode]

Summary

• reorganize settings into focused model, Agent, browser, OSS, GitHub, environment, automation, and security components
• configure Provider ID, optional Base URL, and API Key together; load searchable models for the selected Provider
• label text/vision/reasoning capabilities and require a vision-capable Reviewer in Provider readiness checks
• add save-and-test actions beside each external dependency and invalidate stale connectivity results after configuration changes
• extract API, Shell, login, form controls, and SettingsPage from the monolithic App component
• replace oversized health cards with a compact status strip and rebalance the login layout
• derive the displayed runtime version from package.json instead of the stale hard-coded value

Verification

• image-native format, lint, and typecheck
• 22 test files / 132 tests passed
• production build passed
• headless server and full console E2E passed
• real runtime loaded the DeepSeek catalog and identified the visual Reviewer model
• desktop login/settings visual review: no blocking issue
• independent code review: no blocking issue
• changed-content Secret audit passed