feat(theme): add media viewer and shared docs controls

[aeei]

Summary

• add a shared Base Nova Dialog viewer for Markdown images and Mermaid diagrams
• keep linked-image navigation, keyboard focus restoration, responsive action visibility, and prose spacing token ownership
• enable semantic local search UI with official Kbd shortcuts
• restore the desktop "On this page" heading with the shared translation
• run all theme Playwright specs in CI
• bump @aeei/docusaurus-theme to 0.1.9

Validation

• Jest: 227/227
• Playwright: 26/26
• theme build
• docs-starter production build
• npm pack dry-run
• focused Prettier
• LSP diagnostics clean
• git diff --check
• App, LinkPie, DeskPie desktop/mobile × light/dark visual audit

Visual evidence

• media viewer: /tmp/manual-media-hybrid/
• media spacing: /tmp/manual-media-spacing/
• search: /tmp/manual-search-preview/
• desktop TOC: /tmp/manual-toc-preview/

User visual approval received before push.

Follow-up

After merge, Deck will vendor the exact 0.1.9 tarball in a separate PR.