feat: squash merge all changes from develop-0.1.75

Squash of 124 commits from the legacy develop branch (develop-0.1.75)
onto a clean v0.1.75 upstream base, to simplify future upstream merges.

Key changes included:
- Refactor scope-level rate limiting to model-level rate limiting
- Antigravity gateway service improvements (smart retry, error policy)
- Digest session store (flat cache replacing Trie-based store)
- Client disconnect detection during streaming
- Gemini messages compatibility service enhancements
- Scheduler shuffle for thundering herd prevention
- Session hash generation improvements
- Frontend customizations (WeChat service, HomeView, etc.)
- Ops monitoring scope cleanup

frontend/src/types/index.ts

@@ -591,9 +591,6 @@ export interface Account {
   temp_unschedulable_until: string | null
   temp_unschedulable_reason: string | null
 
-  // Antigravity scope 级限流状态
-  scope_rate_limits?: Record<string, { reset_at: string; remaining_sec: number }>
-
   // Session window fields (5-hour window)
   session_window_start: string | null
   session_window_end: string | null