Vouch Request: Fix font-thicken for IME preedit text (#13758)

[helium777]

What do you want to change?

I'd like to fix the issue described in #13758 (macOS: IME preedit text ignores font-thicken).

Proposed solution: In src/renderer/generic.zig (addPreeditCell), pass .thicken = self.config.font_thicken and .thicken_strength = self.config.font_thicken_strength to self.font_grid.renderCodepoint(...).

I have a patch ready and tested against main, and would be happy to submit a PR once vouched.

Why do you want to make this change?

When font-thicken = true is enabled on macOS, composing text is noticeably thinner than surrounding text because addPreeditCell omits the thickening options. As detailed in #13758, this creates a distracting weight jump upon committing for all IME users (CJK, Hangul, etc.).

I acknowledge that:

• I have read the Contributing Guide and understand the contribution process.
• I have read and agree to follow the AI Usage Policy.
• I wrote this vouch request myself, in my own voice, without AI generating it.

[jcollie]

!vouch