fix: prevent IME Enter from sending messages and refine model vision toggle

[Asunfly]

Summary

• prevent the sidepanel composer from sending on Enter while an IME composition is still active
• add a focused regression test for the Enter submission logic with WXT/Vitest setup
• refine the model vision toggle thumb spacing so it matches the existing settings toggle style more closely

Test Plan

• npm test -- utils/enterSubmit.test.ts
• npm run compile
• npm run build
• manually verified that pressing Enter during Chinese or mixed Chinese/English IME composition confirms input instead of sending
• manually verified the model vision toggle thumb spacing in the options page

Before / After

IME Enter issue

Before:

After:

Notes

• the existing duplicated import and chunk-size warnings from the project build are unchanged by this PR