fix(landing): ship favicon + icons to prod, fix X share preview

[johnnichev]

Summary

• Fix 404s on favicon and og-image in production — deploy workflow now copies landing/favicon.svg, favicon.ico, apple-touch-icon.png, and landing/assets/* into the built MkDocs site. This was the root cause of the missing X/Twitter share card and the missing browser-tab favicon.
• Add apple-touch-icon.png (180×180) so iOS Home Screen / Safari pinned tab show the [ • ] brand mark instead of a screenshot.
• Add favicon.ico (16/32/48 multi-size) as a legacy fallback for older browsers and Windows tile cache.
• Add og:image:secure_url meta tag — cheap insurance for older Slack/LinkedIn unfurlers that sometimes need it even when og:image is already HTTPS.
• Collapse 3 wordmark variants to just variant 3 (terminal banner art) — removes ?logo= URL param, localStorage persistence, data-logo attribute machinery, and ~85 lines of variant-switching CSS + markup.
• New reproducible icon generator: scripts/build_favicons.py redraws the mark from primitives (rectangles + ellipse) rather than rendering the SVG, so CI doesn't need librsvg, cairosvg, or JetBrains Mono installed.
• Tidy-up: adds .playwright-mcp/ to .gitignore (tool artifact directory).

Test plan

• Deploy Documentation workflow runs green on merge
• curl -I https://selectools.dev/favicon.svg → HTTP 200 (currently 404)
• curl -I https://selectools.dev/favicon.ico → HTTP 200
• curl -I https://selectools.dev/apple-touch-icon.png → HTTP 200
• curl -I https://selectools.dev/assets/og-image.png → HTTP 200 (currently 404)
• Share https://selectools.dev/?v=3 on X — confirm share card renders with the 1200×630 og-image + tagline (note: the ?v=3 cache-bust forces X to re-crawl past its stale cache)
• Open https://selectools.dev/ in iOS Safari → Share → Add to Home Screen → confirm the [ • ] icon appears (not a screenshot)
• Visually confirm the nav wordmark still renders as the 3-row box-drawing terminal art
• python3 scripts/build_favicons.py runs clean from a fresh clone (reproducibility check)

Notes

• Squash-merge to match the (#NN) house convention on main.
• Twitter/X caches share cards aggressively. Even after merge, the first share of the bare selectools.dev URL may still pull the old (empty) card. Append ?v=3 (or any query string) to force a fresh twitterbot fetch.