docs(plan): add chat state machine refactor design

Update implementation_plan.md with a detailed refactor plan that splits
`chatInitActor` into two independent actors (`loadQuotaActor` +
`loadHistoryActor`) and redesigns the history loading flow to follow
local → network → save semantics so the UI sees local data first,
then network data.

Key changes outlined in the plan:
- Add new events: `ChatQuotaLoaded`, `ChatHistoryLocalLoaded`,
  `ChatHistoryNetworkLoaded`, `ChatHistorySyncDone`
- Remove dead-code `ChatInit` event
- Extend `ChatState` with `quotaLoaded` and `historyLoaded` flags
- Use an `always` barrier in `guestSession.initializing` and
  `userSession.initializing` so both tasks must complete before
  transitioning to `ready`
- Guest init runs `loadQuota` + `loadHistory` in parallel; non-guest
  init runs `chatWebSocket` + `loadHistory` as independent tasks

Also adds `implementation_plan` to .gitignore.

src/data/dto/chat/guest_chat_quota.ts

@@ -12,6 +12,7 @@ export class GuestChatQuota {
   static readonly maxQuotaPerDay = 40;
   static readonly maxQuotaPerDayTest = 4;
   static readonly quotaWarningThreshold = 20;
+
   static readonly quotaWarningThresholdTest = 2;
   static readonly defaultTotalQuota = 50;
   static readonly defaultTotalQuotaDev = 5;

src/stores/chat/chat-machine.helpers.ts

@@ -96,14 +96,6 @@ export function mapTotalQuotaResult(
   return fallback;
 }
 
-// ============================================================
-// Init 任务 1：游客配额（**纯** local）—— `loadQuotaActor` 调
-// ============================================================
-/**
- * 读取游客配额（**仅** guestSession 有意义）
- * - 游客**日**配 + 总**配** 两**项**都**从** ChatStorage 拉（**纯** local，**无**网络）
- * - 失**败**返 fallback 0（**不**抛）—— **让** UI **还**是**能**进** ready
- */
 export async function readGuestQuota(): Promise<{
   remaining: number;
   total: number;

src/stores/chat/chat-machine.ts

@@ -1,13 +1,6 @@
 /**
  * Chat 状态机（XState v5）
  *
- * 设计要点：
- *   - 使用 XState v5 `setup({...}).createMachine({...})` 声明式 API
- *   - HTTP 操作用 `fromPromise` actor（一次性 Promise）
- *   - WebSocket 用 `fromCallback` actor（长生命周期）—— **机器内**完整管理
- *   - chat-screen **不**直接管理 WS —— 只派 3 个鉴权生命周期事件
- *   - 保持事件类型名与原 Dart 一致，便于业务层迁移对照
- *
  * **鉴权解耦**（**事件驱动**）：
  *   chat 机器**不**感知鉴权 / **不**管 WebSocket —— 由 chat-screen 派生 loginStatus
  *   chat-screen **只**派 3 个事件：
@@ -308,14 +301,44 @@ export const chatMachine = setup({
         },
         ready: {
           on: {
-            ChatSendMessage: {
-              actions: ["logSendMessageReceived", "appendUserMessage"],
-              guard: ({ event }) => event.content.trim().length > 0,
-              target: "sending",
-            },
-            ChatSendImage: {
-              actions: ["logSendImageReceived", "appendUserImage"],
-            },
+            // **配**额**检**查** + 发**送**消息**（3 **条** guard，**按**顺**序**匹**配**）
+            // 1. **总**配**额**（guestTotalQuota）**不**足 → **触**发** quota exceeded dialog，**不**发**送**
+            // 2. **每**日**配**额**（guestRemainingQuota）**不**足 → **同**上**
+            // 3. **两**个**配**额**都** OK + **内**容**非**空** → **发**送** + **减**配**额**
+            ChatSendMessage: [
+              // 1. 总配**额** **不**足**（guestTotalQuota <= 0） → **触**发** quota exceeded dialog
+              {
+                guard: ({ context }) => context.guestTotalQuota <= 0,
+                actions: "incrementQuotaExceeded",
+              },
+              // 2. 每**日**配**额** **不**足**（guestRemainingQuota <= 0） → **触**发** quota exceeded dialog
+              {
+                guard: ({ context }) => context.guestRemainingQuota <= 0,
+                actions: "incrementQuotaExceeded",
+              },
+              // 3. 两**个**配**额**都** OK + **内**容**非**空** → **发**送**
+              {
+                actions: ["logSendMessageReceived", "appendUserMessage"],
+                guard: ({ event }) => event.content.trim().length > 0,
+                target: "sending",
+              },
+              // 兜**底**：**内**容**空** / 什**么**都**不**匹**配** → **静**默**丢**弃**
+            ],
+            // **配**额**检**查** + 发**送**图**片**（**同**样** 3 **条** guard，**不**检**查** "内**容**空"**）
+            ChatSendImage: [
+              {
+                guard: ({ context }) => context.guestTotalQuota <= 0,
+                actions: "incrementQuotaExceeded",
+              },
+              {
+                guard: ({ context }) => context.guestRemainingQuota <= 0,
+                actions: "incrementQuotaExceeded",
+              },
+              {
+                actions: ["logSendImageReceived", "appendUserImage"],
+                target: "sending",
+              },
+            ],
             // **删**除 ChatLoadMoreHistory handler —— 游客**无**服**务**端** history，**不**支**持**翻**页**
             ChatLogout: "#chat.idle",
             ChatNonGuestLogin: "#chat.userSession",