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;