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/UI_DESIGN.md

@@ -142,15 +142,18 @@ Login uses `Screen`.
 - **TapConfirm** (`components/TapConfirm.svelte`, logic in `lib/tapconfirm.ts`): the shared
   tap-to-confirm control. A first tap arms a ~2 s window showing a **fading ✅** (no fade
   under reduce-motion, but the window still holds); a tap on the ✅ within it confirms,
-  otherwise it reverts. Used by the **Skip** and **Hint** tabs (the icon morphs to ✅, no
-  label — replacing the old press-and-hold popover) and the in-game **add-friend 🤝**. The
+  otherwise it reverts. Used by the **Hint** tab (the icon morphs to ✅, no label — replacing
+  the old press-and-hold popover) and the in-game **add-friend 🤝**. The
   **Drop game** action keeps its `Modal` confirmation (a destructive, less-frequent action).
 - **MakeMove / Reset**: when ≥1 tile is pending the rack collapses its used slots
   and shifts left, a **borderless ✅ icon button** (styled like a tab, not a filled accent
   button) beside the rack commits the move — no popover, and disabled while the pending word
   is known illegal; the 🔀 Shuffle tab is replaced by a **↩️ Reset** tab.
-- **Game tab bar**: 🔄 Draw (disabled when the bag is empty), 🥺 Skip, 🛟 Hint (with a
-  remaining-count badge, disabled at zero); 🔀 Shuffle (no label, no confirm), which
+- **Game tab bar**: 🔄 Exchange/Pass (opens a dialog — pick tiles to **Exchange N**, or pick
+  none to **Pass without exchanging**; pass is always available on your turn, exchange only
+  while the bag still holds a full rack, below which the tiles disable and only the pass
+  remains), 🛟 Hint (with a remaining-count badge, disabled at zero); 🔀 Shuffle (no label,
+  no confirm), which
   **animates** — tiles hop along a low parabola to their new slots (duration scaled by the
   distance, the longest ≤ 0.3 s; off under reduce-motion) with a short haptic shake. The
   under-board slot shows the **Scores: N** preview. The screen **title** is the variant's