#16 Favicon + head standards (manifest, og, theme-color)

Context: Favicon + head standards. Blocked by #15 (needs logo.svg). Tasks:

1. static/favicon.svg (logo adapted) + favicon.ico? (ICO needs binary — prefer SVG-first: <link rel=icon type=image/svg+xml>, plus apple-touch-icon.png? PNG needs generation — use SVG + documented fallback; if PIL/canvas unavailable, ship svg + 180px svg apple-touch reference. Decide in-work, keep zero-dep: hand-write SVG only, no binary assets.)
2. Head standards in layout: favicon links, apple-touch-icon, theme-color (#1a1b26 dark / light per #18?), og:title/description/type, manifest.webmanifest (name, short_name hu13, icons svg, display minimal, theme colors). Serve .webmanifest with correct content-type in serve (mime map addition, test it).
3. Tests: head asserts on fixture build (icon/manifest/theme-color/og present), manifest parses as JSON, mime test for .webmanifest. Success rubric:

• All head tags present in every page (layout-level); manifest valid JSON; no binary blobs in repo. Swarm: Owlette. Branch m6/favicon. Merge after #15.