fix(client): guard additionalCostChoices Choice branch against a short payload

[jeffrey701]

Problem

additionalCostChoices (client/src/viewmodel/costLabel.ts) formats the Choice branch by indexing straight into the cost tuple:

case "Choice":
  return { title: "Choose additional cost", options: [
    { id: "pay", label: formatAbilityCost(cost.data[0]) },
    { id: "decline", label: formatAbilityCost(cost.data[1]) },
  ] };

data is typed [SerializedAbilityCost, SerializedAbilityCost], but it is deserialized from the engine. A malformed/short payload leaves cost.data[1] undefined; formatAbilityCost switches on cost.type, so undefined.type throws a TypeError and hard-crashes the cost-choice modal instead of degrading. The sibling Kicker branch already guards exactly this (const label = first ? … : "kicker").

Fix

Destructure and guard each option with a fallback label ("Pay" / "Decline").

Test

client/src/viewmodel/__tests__/costLabel.test.ts — a well-formed two-cost Choice still labels both options; a short payload no longer throws and falls back to "Decline".

[gemini-code-assist]

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