feat(ui): merge Exchange/Pass into one action; drop dead Tournaments tab

Lobby: remove the inert 🏆 Tournaments tab (it only raised a 'coming soon' toast); the lobby is back to three tabs, matching docs/FUNCTIONAL.md.

Game: fold the separate 🥺 Skip (pass) tab into the 🔄 tab, now Exchange/Pass. Its dialog passes when no tile is selected (button 'Pass without exchanging') and exchanges when tiles are ('Exchange N'). The tab is no longer gated on an empty bag (pass must stay reachable in the endgame); inside the dialog tile selection is disabled while the bag is below a full rack (bagLen >= RACK_SIZE).

The merge is UI-only. A pass is NOT an exchange of zero tiles: the rules allow an exchange only with a full rack left in the bag and forbid a zero-tile swap, while a pass is always legal; GCG (Poslfit) writes a pass as a bare '-' and an exchange as '-TILES'. Pass and exchange stay distinct end-to-end (wire, engine, history/GCG); the dialog dispatches the existing gateway.pass / gateway.exchange. No backend/wire/history/GCG change.

Docs: docs/UI_DESIGN.md, docs/FUNCTIONAL.md (+_ru), PRERELEASE.md. Tests: ui/e2e/game.spec.ts.

docs/FUNCTIONAL.md

@@ -102,8 +102,10 @@ settings and the game starts once every invitee has accepted — any decline can
 expires after seven days.
 
 ### Playing a game
-Place tiles, pass, exchange, or resign. Tiles are laid without choosing a
-direction — the game infers the play's orientation, so a single tile that extends
+Place tiles, pass, exchange, or resign. Pass and exchange share one control —
+choosing no tiles passes the turn, choosing tiles exchanges them. Tiles are laid
+without choosing a direction — the game infers the play's orientation, so a single
+tile that extends
 an existing word (down a column or across a row) is accepted. A play is validated
 against the game's dictionary at submit time and scored; an unlimited preview
 reports the word(s) a tentative move would form and its score, or that it is not