feat(desktop-settings): clean up copy and layout

[mahanti]

Summary

• Constrain Settings content to a focused 640px column and simplify section copy.
• Add an interactive Appearance workspace preview for theme, density, links, and thread layout.
• Coordinate link-preview and thread-layout editing with the preview thread panel using the same transition as direct interaction.
• Standardize Settings action buttons, toggle motion, accent selection, and notification controls.
• Make sound previews complete reliably with waveform progress and isolated playback.
• Expand focused E2E coverage for layout, motion, reduced motion, sound, and shared controls.

Screenshots

Appearance

Before	After

Agent defaults

Before	After

Test plan

• TypeScript typecheck.
• Desktop unit test suite.
• Desktop formatting, lint, zoom-accessibility, attribution, sign-off, and secret-scanning checks.
• Focused Playwright coverage for Appearance, Notifications, Local Archive, and Agent Defaults.
• E2E Vite build.