repo: dripnex/plugin-math — extract math/LaTeX rendering

[tomymaritano]

Context

src/renderer/plugins/math.tsx (166 LOC, KaTeX block + inline) becomes a satellite consuming the markdown renderer extension points.

Acceptance criteria

• Repo from plugin-template; npm @dripnex/plugin-math; runtime id dripnex-math
• Uses ONLY @dripnex/plugin-api (markdown.registerBlockRenderer for ```math fences + inline renderer) — CI boundary enforced
• Desktop preinstalls it via PluginRegistry; hardcoded math.tsx deleted
• Toggleable in settings; math still renders identically (visual check + e2e block-render assertion)

Depends on

Markdown extension points + publish pipeline (Q1).