feat(chat): handle credit-gated send responses

src/stores/chat/chat-machine.ts

@@ -206,6 +206,10 @@ export const chatMachine = setup({
     clearUpgradePrompt: assign(() => ({
       upgradePromptVisible: false,
       upgradeReason: null,
+      canSendMessage: true,
+      cannotSendReason: null,
+      requiredCredits: 0,
+      shortfallCredits: 0,
     })),
 
     markPaymentUnlockPending: assign(() => ({
@@ -306,7 +310,8 @@ export const chatMachine = setup({
           on: {
             ChatSendMessage: {
               actions: ["appendGuestUserMessage", "enqueueMessage"],
-              guard: ({ event }) => event.content.trim().length > 0,
+              guard: ({ context, event }) =>
+                context.canSendMessage && event.content.trim().length > 0,
             },
             ChatSendImage: {
               actions: "appendGuestUserImage",
@@ -441,7 +446,8 @@ export const chatMachine = setup({
         ready: {
           on: {
             ChatSendMessage: {
-              guard: ({ event }) => event.content.trim().length > 0,
+              guard: ({ context, event }) =>
+                context.canSendMessage && event.content.trim().length > 0,
               actions: ["appendUserMessage", "enqueueMessage"],
             },
             ChatSendImage: {