feat(plugins): customize DeepSeek answer typography

Proposal

Add answer-only font size, line height and paragraph spacing as one coherent reading feature. Neutral defaults preserve native rendering; excludes code, formulas, thinking content and composer. Settings and labels in all ten locales, reversible lifecycle, narrow/desktop and light/dark verification.

Contribution workflow

The contributor requests implementation where feasible. This issue records scope before code. Maintainer confirmation of new primitive contracts is still needed; no maintainer approval is implied by opening this issue. New PRs will be submitted only after applicable checks pass, with real-browser gaps resolved rather than labelled as complete. Related DeepSeek integration: #960 and #995.