webclient: draw the PWA icons at full size again ...

The maskable icons and the apple-touch-icon carried the mark at 21% of the
canvas — the artwork scaled down and pasted in the centre — so on a home
screen the logo read as a fragment of itself. The mark takes 73.4% of the
canvas in logo.svg and in the launcher tile of the Android app icon; that is
the proportion all five files use now.

scripts/gen_pwa_icons.py redraws the mark from logo.svg's geometry with
Pillow (already a project dependency, no SVG rasterizer needed) and writes the
whole set, so the scale lives in one constant; --check measures what is on
disk and reports SUSPECT if it drifts again. Two runs produce identical bytes.

The regenerated icon-192/512 are geometrically identical to the rsvg-rendered
ones they replace: ink bbox 376x376 with 68px insets at 50% coverage, and the
same ink mass across the stroke. Only the antialiasing bytes differ.

dist/ rebuilt (it carries a copy of public/ and is served by the backend).

Tests: frontend 148 passed; backend 1367 passed, 1 skipped.