fix(desktop): clip sidebar overflow during expansion

[saltand]

Summary

Prevent the sidebar's expanded content from briefly creating a horizontal
scrollbar while the sidebar width transition catches up.

Verification

• npx biome check apps/desktop/src/renderer/styles/shell-layout.css
• npm --workspace @maka/desktop run typecheck

Before

recording_2026-08-25_17-11-23.mp4

After

recording_2026-08-25_17-04-11.mp4

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope:
Codex assisted with diagnosing the transient overflow and implementing the CSS fix.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[Astro-Han]

I reviewed this head and found no blocking issues.

The change is 2 lines in shell-layout.css (overflow: clip on the width authority) for the sidebar motion. No overlay/clip regression, and the typecheck/build blocks are baseline mismatches unrelated to this change.

No P0-P3.

简体中文

该头未发现阻断。

[Astro-Han]

APPROVE — 5ce136c, no P0-P3, CSS clip correctly scoped.