Replace the vole faces with an abstract mark

Replace the vole faces with an abstract mark

The faces were too literal for the rest of the interface. A name now
gets one curled stroke instead: a burrow turned in on itself, a fern
frond, a shell, whichever the reader sees. It never says which.

The point is what it does not do. It is one cream line on one of seven
earthy grounds, with the same round caps and stroke language for
everyone, so it is quiet next to a name and still legible at 17px. A few
shapes were drawn and dropped on the way: a crescent is elegant but
barely changes between names, arches read as rainbows or smiles, and
rings, mosaics and tally marks read as icons. A curl carries enough
variation (how far it winds, tightness, direction, rotation, thickness,
position) to tell people apart without looking like a diagram.

The derivation is unchanged: SHA-256 over the name, nothing stored, no
upload, no avatar service. The path is marked `fill="none"` in the
markup; the sheet owns the stroke colour, so the palette stays a theme
decision.