Add appearance preference previews

[morgmart]

Summary

• Replace the Link previews and Thread layout dropdowns with the shared segmented control from Add font size and conversation density preferences #5644.
• Pair each setting with a real, inert preview: production Compact/Rich link cards and a simplified Focus/Split thread diagram.
• Keep scrub previews local to Settings so only a completed selection updates the saved preference and real app behavior.
• Keep each setting and preview responsive as one grouped unit, with focused light/dark and narrow/wide coverage.

Dependency

• Stacked on Add font size and conversation density preferences #5644, which provides the typography/density foundation, shared SegmentedControl, and responsive SettingsOptionRow.
• After Add font size and conversation density preferences #5644 merges, this PR should be retargeted/rebased onto main.

Validation

• pnpm typecheck
• pnpm check:px-text
• pnpm check:file-sizes
• pnpm exec playwright test tests/e2e/appearance-previews.spec.ts --project=smoke (3 passed)
• Focused existing Appearance coverage (2 passed)
• Pre-push desktop checks, typecheck, tests, and Tauri checks

[morgmart]

Rich link preview — dark, narrow

The grouped setting stacks responsively and uses the real rich-card presentation without interactive affordances.

Focus thread layout — dark, narrow

The foreground thread panel overlaps the channel surface with rounded leading corners.

Compact link preview — light, wide

The compact production card stays paired with its setting at desktop width.

Split thread layout — light, wide

The split diagram preserves the side-by-side relationship and shared visual language.