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.

webclient: PWA hardening — stable height skeleton, touch selection policy, fixed maskable icons ...

- html/body/#app height via percentages instead of 100vh/dvh: the standalone
  PWA visual viewport drifts with keyboard/system panels, which let the
  document exceed the screen and scroll fixed headers away.
- viewport meta gains interactive-widget=resizes-content; phone-width inputs
  (textarea/text) are pinned to 16px to avoid mobile focus zoom.
- Selection policy: service chrome (chat header, sidebar, tab strip, input
  bar, meta rows) is unselectable with -webkit-touch-callout, killing the
  'Search with Google' sheet on long-press; message content and settings
  panels keep selectable text.
- Apple touch icon switched to a proper 180px tile.
- Regenerated maskable icons (were degenerate 1px-tall files): dark
  background plus the logo inside the 80% safe zone.
- manifest gains display_override.