fix(workspace): improve preview panel sizing

[Dustella]

Summary

• make the preview panel default to 56% of the available workspace while preserving room for the main content
• raise the safe preview width ceiling to 1440px and react to window/sidebar size changes
• add expand/restore controls, double-click toggling, keyboard resizing, and a larger resize hit area
• preserve stored user widths and cover the width calculations with unit tests

Closes #30

Testing

• pnpm --filter @oh-my-github/client test (41 files, 230 tests)
• pnpm typecheck
• pnpm --filter @oh-my-github/client build
• Playwright against the built Electron app at default and ultra-wide window sizes, including light/dark rendering, expand/restore, drag persistence, keyboard resize, and double-click restore