Refactor settings UI tabs

[danshapiro]

Summary

• reorganizes Settings into focused tabs for appearance, coding agents, panes, workspace, naming, network, and advanced controls
• moves coding agent enablement into compact per-agent rows backed by current freshAgent settings
• preserves current fresh-agent/Codex display-turn behavior while rebasing the older settings UI branch

Verification

• npm run lint (0 errors, existing warnings)
• npm run typecheck
• npm run test:vitest -- run test/unit/client/components/SettingsView.agent-chat.test.tsx test/unit/client/components/panes/PanePicker.test.tsx
• npm run test:vitest -- run test/unit/server/fresh-agent/codex-normalize.test.ts --config vitest.server.config.ts
• npm run test:vitest -- run test/unit/client/components/fresh-agent/FreshAgentComposer.test.tsx
• FRESHELL_TEST_SUMMARY='new-settings-ui rebased PR verification rerun' npm run check