Move settings entry from title bar to sidebar footer

[xuy]

Summary

• move the settings entry out of the macOS title bar and into a persistent sidebar footer button
• replace the right-side settings drawer with a full settings view in the main content area
• keep settings reachable when the sidebar is collapsed and update focused UI coverage

Testing

• pnpm --filter @noah/desktop exec vitest run src/components/ui.test.tsx --silent
• pnpm --filter @noah/desktop exec tsc --noEmit