webclient: settings tab polish, light chat-table styling, iOS launch screens ...

- Synapse tab icon was a no-op glyph: ph-diagram-project does not exist in
  the bundled Phosphor regular set — the button rendered bare text. Switched
  to ph-network, which ships.
- .settings-tabs capped at 1200px width on wide screens (was max-width:none).
- Chat markdown tables: the old .msg-assistant-content .table rules were
  dead (markdown never adds a .table class) — retarget .table-wrap table and
  restyle lightly: framed border + radius, muted header row, plain row
  separators, no uppercase/no hover color jump.
- iOS home-screen: apple-mobile-web-app metas and a full apple-touch-startup-
  image set (24 device classes, portrait+landscape) rendered from logo.svg
  on the theme background.