refactor(ui): migrate Markdown core to Astryx

[Astro-Han]

Summary

Refs #1565.

• Replace Streamdown plus Maka's parallel prose/link/code CSS with Astryx Markdown as the single parser, typography, table, task-list, and syntax-highlighting authority.
• Preserve Maka's product trust boundaries: redact before the Suspense fallback, keep the typed maka: navigation allowlist separate from the http/https image policy, and prevent non-allowlisted Markdown images from loading.
• Keep stream pacing and scroll anchoring on the existing conversation path for PR 8 while applying Astryx's pure incomplete-syntax repair before parsing.
• Retain localized success/failure feedback around Astryx 0.1.9's code-copy button because that release discards clipboard rejections.
• Remove the retired Streamdown dependency closure and generated license inventory, and add one composed-shell Storybook state for Markdown review.

Verification

• npm --workspace @maka/ui test — 280 passed.
• npm --workspace @maka/desktop test — 2966 passed.
• npx playwright test --config e2e/playwright.config.ts e2e/send-message.spec.ts — 2 passed, including exact copied code and clipboard rejection feedback.
• npm run rebuild, npm run typecheck, npm run lint, npm run format:check, and npm run check:release — passed.
• npx knip --workspace packages/ui and npx knip --workspace apps/desktop — passed (desktop reports only the existing electron-builder configuration hints).
• npm --workspace @maka/desktop run build-storybook — passed.
• npm run check:visual-contract -- --against main --scopes scripts/visual-contract-scopes/pr7-markdown-core.json — 20/20 route, theme, and platform comparisons clean; 48 Markdown/turn-flow changes declared where the chat fixture is present and zero changes outside the declared subtree.
• npm run check:hit-test — 5/5 routes clean.

Review focus

• Astryx 0.1.9 bypasses components.image for standalone image blocks. The pre-parse guard closes that upstream gap while leaving fenced and inline code untouched; tests cover direct, inline, and reference images.
• Astryx 0.1.9 catches clipboard write failures without exposing an error callback. A capture-boundary adapter delegates the write to Maka's existing clipboard-feedback owner and can be removed once Astryx exposes that callback.
• scripts/visual-contract-scopes/pr7-markdown-core.json is the issue-required declaration for this migration slice; the final migration cleanup PR removes the harness.
• package-lock.json and apps/desktop/resources/licenses/npm/THIRD_PARTY_NOTICES.txt are generated reductions from removing Streamdown's production dependency closure.

Visual evidence

Live Electron shell, light theme (deterministic E2E fixture):