Katan: open trade offers and counter-offers

- Offers can go to the whole table (asked in seat order, empty hands skipped) or one seat.
- The asked seat may accept (trade happens at once), decline, or counter with new terms.
- If nobody accepts and there are counters, the offerer takes one or turns them all down (new TRADE_COUNTERS phase).
- AI: fixes an inverted accept test (it weighed what it gives against what it gets backwards), picks the best counter.
- UI: Everyone option in the offer dialog, Counter… in the reply dialog, a counter-choice dialog.
- Tests and docs updated.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_018njn2eMPAYqAoE49hGRg2X

docs/UI-catan.md

@@ -89,7 +89,8 @@ screen's UI state. The legal-move list stays the gate.
 | Seven | **Discard** dialog (forced); then taps a hex to move the robber; a victim dialog appears only if more than one player can be robbed |
 | Main turn | arms **Road / Settlement / City** and taps a highlighted spot; **Buy card**; **Play card**; **Trade**; **End turn** |
 | Road Building | taps highlighted edges, or **Done with roads** |
-| Offered a trade | **Accept / Decline** dialog (forced) |
+| Offered a trade | **Accept / Decline / Counter…** dialog (forced); *Counter…* opens a picker pre-filled with the mirror of the offer |
+| Counter-offers received | **Counter-offers** dialog: *Accept* beside the one to take, or *No deal* |
 | Game over | result dialog |
 
 Trade offers are free-form (two card multisets), so they are validated with
@@ -137,6 +138,6 @@ Not done yet, in rough priority order:
    it does not say which hexes paid out, so that needs either a diff of two states
    or an engine addition.
 3. Art: terrain textures, resource and harbour icons, a real robber pawn.
-4. Open trade to the whole table and counter-offers (an engine limitation too).
+4. A timer or cancel button on offers (the engine has neither).
 5. A board that shows the *last* move (a faint highlight of what just changed).
 6. Landscape polish on large tablets; the right-hand panel scrolls but is plain.