fix(#421): render safe blog tables and blockquotes

[TortoiseWolfe]

Closes #421

Summary

• replace the blog regex HTML renderer with a synchronous, blog-specific safe GFM pipeline
• render semantic tables and blockquotes while preserving safe authored HTML, legacy heading and TOC anchors, external-link behavior, and Prism copy markup
• add keyboard-reachable mobile table overflow styling and exact 390px and 500px browser regressions for both affected posts

Validation

• focused renderer, processor, TOC, component, and accessibility suites: 143/143
• root-path static-export Playwright regression: 2/2
• negative controls: removing GFM fails semantic table coverage; clipping wrapper overflow fails real 390px geometry coverage
• pre-push gate: gitleaks, lint, type-check, breakpoints, 138-page production build, 107 emitted chunk parses, and first-load budget