Add a self-contained icon system: vendored Material Symbols registry, an
<Icon> component that references a single <symbol> sprite via <use>, plus
IconButton/IconSprite. Replace every emoji/Unicode glyph (logo, theme
switcher, cover placeholder, like/comment counts, lightbox controls, back
links) and add leading icons to key controls. Decorative icons are
aria-hidden; icon-only buttons get labels; theme toggles expose
aria-pressed; counters get accessible names.
Recorded in openspec change material-icons-ui (all tasks complete).
iu
997ec5e6c1