ADD explains itself instead of dead-ending

Two numbers for movement work by playing them one after another, the
Add spending itself on the second — but clicking the ADD card itself,
the natural first instinct, showed nothing but a cancel button. It
now says how the ride works; a second number without an Add in hand
says what is missing instead of offering a doomed button; and the
button's label says "spends your Add" at the moment it does.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/web/src/App.svelte

@@ -1681,10 +1681,20 @@
                 clearSelection();
               }}>Drop it here</button>
             {/if}
+            {#if selectedCard?.cardId === "add"}
+              <span>— ADD rides a second NUMBER: play one number for movement,
+                then play another, and the Add spends itself</span>
+            {/if}
             {#if selectedCard && isNumberCard(selectedCard.cardId)}
-              <button class="stamp tiny" onclick={playNumberForMovement}>
-                Play for +{cardDef(selectedCard.cardId).value} movement{view.turn.numberPlayedForMovement ? " (uses your Add)" : ""}
-              </button>
+              {@const needsAdd = view.turn.numberPlayedForMovement}
+              {@const haveAdd = view.yourHand.some((c) => c.cardId === "add")}
+              {#if needsAdd && !haveAdd}
+                <span>— a second movement number needs an ADD card</span>
+              {:else}
+                <button class="stamp tiny" onclick={playNumberForMovement}>
+                  Play for +{cardDef(selectedCard.cardId).value} movement{needsAdd ? " (spends your Add)" : ""}
+                </button>
+              {/if}
             {/if}
             <button class="hint-cancel" onclick={clearSelection}>cancel</button>
           {/if}