refactor(frontend): one Icon component, color inherited, one 14px square frame

[Kadrian]

Design-system prep, step 5: one Icon in a square frame, color inherited

Stacked on #4002. icon/FaIcon with its twelve flags becomes ui-components/Icon, a thin wrapper over FontAwesomeIcon: explicit icon imports, one component, one frame, color from currentColor.

• every icon sits centered in a 14 by 14 px frame (size-3.5), so icons line up in rows, lists and menus regardless of glyph shape; the few picture-like icons use a size-* class (the stats block and the preview spinner at 30 px)
• color is inherited from the surrounding text; a button colors its icon through its own text color, so IconButton's second color table is gone
• Font Awesome's stylesheet is imported into tailwind's base layer instead of being injected at runtime, so size utilities apply to icons and the w-…! overrides go
• the color and spacing flags became classes at the 80 call sites; large, small and fixedIconWidth on icons are gone
• Icon stories: the frame with narrow and wide glyphs, a picture-size icon, color inheritance, spinner

Worth a look: icons that had no flag were hard gray-800 and now follow their text; the fourteen icons that were 16 px are 14 px like all others.