Avoid native line-through on task titles

[Tillingxianyu]

Summary

Fixes #10 by avoiding native line-through on completed task titles. Safari can choke on that decoration when the title contains a complex ZWJ emoji sequence like 👨‍👩‍👧‍👦, so the completed-state line is now drawn with CSS instead of decorating the text node directly.

Verification

• Opened the app locally
• Added Plan trip 👨‍👩‍👧‍👦
• Toggled it done
• Confirmed the task stayed visible and no console errors were logged

[codebounty-dev-bot]

⚠️ Attention Sponsor: This PR was submitted by @Tillingxianyu who has not applied for this bounty (#10). Please verify if you want to consider this contribution.

👋 Hey @Tillingxianyu: This issue has a bounty on CodeBounty! To be eligible for the reward, please sign up on CodeBounty and apply for this bounty. If you apply, your PR will be automatically associated with your application.

[Tillingxianyu]

I have signed up on CodeBounty and applied for this bounty(#10).Please take a look whe you are free, thank you!

[Tillingxianyu]

Hi, I found bounty #10 on CodeBounty, but it is currently marked as "Abandoned" and there is no Apply/Claim button available.
My PR #17 fixes the original issue #10. Could you please confirm whether this bounty can still be associated with my PR, or whether it is no longer eligible for payout?