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.