gpui: Fix `line_through`, `underline` position when used text center or right (#24721)

Release Notes:

- N/A

---

| Before | After |
| --- | --- |
| <img width="912" alt="image"
src="https://github.com/user-attachments/assets/0640ac85-ee5d-4707-b866-997e36608c18"
/> | <img width="912" alt="image"
src="https://github.com/user-attachments/assets/caf84477-a7bc-4c22-a9e6-f44c3b6f86ef"
/> |
 
And fix the `line_through` doc link.