Add persistent Settings account sidebar

[shaunandrews]

Related issues

• Related to STU-2105
• Stacked on Unify Settings cards #4543.
• Extracted from the design exploration in Design Exploration: Redesign global app settings: two tabs, a shared design system, and an account sidebar #4316.

How AI was used in this PR

Codex isolated the persistent account context from the larger settings information-architecture change, adapted it to the shared cards introduced lower in the stack, updated focused tests, and verified the layout at wide and narrow widths in both color schemes. The final diff and rendered UI were reviewed interactively.

Proposed Changes

• Keep WordPress.com identity, authentication actions, documentation, and issue reporting visible from every Settings tab.
• Move Account out of the long General settings column into a dedicated sidebar on wide windows.
• Keep the sidebar in view while the main settings column scrolls, with Help pinned to the bottom on wide windows.\n- Keep the sidebar unframed, matching the original Design Exploration: Redesign global app settings: two tabs, a shared design system, and an account sidebar #4316 exploration, so only the main settings content reads as cards.
• Stack Account and Help above the active settings content on narrower windows, with a single-column fallback on small windows.

This PR intentionally keeps Usage as its own tab and preserves all six tabs. It improves account and support discoverability on its own without depending on the final navigation consolidation.

Screenshots

Light	Dark

Testing Instructions

1. Check out this branch with Unify Settings cards #4543, Add Agent Instructions toggle #4542, and Improve Settings usage meters #4540 beneath it.
2. Run npm run cli:build:ui && node apps/cli/dist/cli/main.mjs ui --no-open.
3. Open Settings and confirm Account and Help appear beside General on a wide window.
4. Visit AI, Usage, Keyboard, Skills, and MCP; confirm Account and Help remain available on every tab.
5. On a long settings tab, scroll the main content and confirm the account sidebar remains visible.
6. Resize below 900px and confirm Account and Help move above the active tab content.
7. Resize below 640px and confirm Account and Help stack into one column.
8. Check the layouts in both Light and Dark appearance settings.

Verified on this branch:

• npx eslint --fix on all modified TypeScript files
• npm run typecheck
• npm test -- apps/ui/src/components/settings-view/account-section.test.tsx apps/ui/src/components/settings-view/index.test.tsx (19 tests)
• npm run cli:build:ui
• Browser verification at desktop and 800px widths in light and dark themes with no console errors

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?