feat(ui): batch of UI polish tweaks

- admin dashboard "Users" count excludes robots (CountUsers, humans only)
- lobby finished-game delete reveal: background matches header/tab-bar (--bg-elev)
- mobile: swipe up on the zoom-out board (no staged tiles) shuffles the rack
- lobby: status icons -25%, game-row top/bottom padding halved
- toast: info tier drifts up ~a tab-bar height while fading over 2s and dismisses
  on tap; error tier unchanged
- game: confirm-move button stays pinned at the right edge; rack tiles shrink to
  fit so a full rack never overflows onto it
- telegram: a successful invite-link redeem shows a welcome window pointing at the bot
- settings: remove the grid-lines toggle; the board is always a gapless checkerboard
- settings/comms hubs: the selected tab highlight wraps icon + label as a pill

Docs: UI_DESIGN (toast, board surface, selected tab), PLAN; e2e updated for the
removed grid-lines toggle.

ui/src/lib/session.ts

@@ -124,8 +124,6 @@ export interface Prefs {
   locale: Locale;
   reduceMotion: boolean;
   boardLabels: BoardLabelMode;
-  /** Draw the 1px grid lines between cells; off (default) shows a gapless checkerboard. */
-  boardLines: boolean;
 }
 
 export async function loadPrefs(): Promise<Partial<Prefs>> {