feat(tui): replace [x] completion marker with [✓] across all sidebar panels

[IIzzaya]

Summary

In tui mode, replace [x] completion marker with [✓] across all sidebar panels (more Chinese style lol)

Testing

• cargo test --all-features
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features

Checklist

• Updated docs or comments as needed
• Added or updated tests where relevant
• Verified TUI behavior manually if UI changes

[gemini-code-assist]

Warning

Gemini encountered an error creating the review. You can try again by commenting /gemini review.

[weikengchen]

:) this sounds like a config option thing as github uses [x] (in Markdown), but I think that is due to historical reason because x is ASCII and ✓ is not.

But ✓ does look better.

[Hmbown]

Thanks for the visual polish PR. The idea is understandable, and the screenshot makes the intent clear, but I am leaving it out of v0.8.42 because this is a product/style decision rather than a stabilization fix.

I agree with the comment that this may want to be a design/config decision instead of a blanket replacement, especially because [x] is ASCII/Markdown-familiar while the check mark reads better visually. Routing this to a later presence/UI pass rather than closing it.