refactor(chat): remove unused ChatScreenVisible and ChatScreenInvisible events

The screen-lifecycle events were dead code: the transitions in chat-machine
(idle <-> ready) had no caller behavior tied to them, and the dispatches
in chat-screen.tsx were the only senders. Drop the union members, both
state transitions, and the mount-effect dispatches.

The idle state remains declared as initial but is no longer reachable
at runtime; can be refactored in a follow-up.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/stores/chat/chat-machine.ts

@@ -251,7 +251,6 @@ export const chatMachine = setup({
     idle: {
       on: {
         ChatInit: "initializing",
-        ChatScreenVisible: "ready",
         ChatLoadMoreHistory: "loadingMoreHistory",
         ChatSendMessage: {
           guard: ({ event }) => event.content.trim().length > 0,
@@ -308,7 +307,9 @@ export const chatMachine = setup({
           actions: "appendUserImage",
         },
         ChatLoadMoreHistory: "loadingMoreHistory",
-        ChatScreenInvisible: "idle",
+        ChatAuthStatusChanged: {
+          actions: "refreshAuthStatus",
+        },
         ChatAISentenceReceived: {
           actions: "appendOrUpdateAISentence",
         },