[eric] [eric] add PostHog analytics, 9Router subscription proxy, unified usage tracking

- PostHog integration: collector, analytics subapp, opt-in UI, Analytics page
  - 9Router: auto-start, OAuth subscription flow, /v1/messages Anthropic format support
  - Settings overhaul: multi-provider API keys, subscription connect UI, onboarding modal
  - Unified usage: merge 9Router cost/token data into Settings Usage tab
  - Provider system: providers/, agent_loop, tools/ (unused, for future non-Anthropic support)
  - Agent SDK: restored as primary with 9Router ANTHROPIC_BASE_URL fallback
  - Updated system prompt, credential resolution, dashboard analytics

frontend/src/shared/ws/WebSocketManager.ts

@@ -284,7 +284,7 @@ class WebSocketManager {
   sendMessage(
     sessionId: string,
     prompt: string,
-    opts?: { mode?: string; model?: string; images?: Array<{ data: string; media_type: string }> },
+    opts?: { mode?: string; model?: string; provider?: string; images?: Array<{ data: string; media_type: string }> },
   ) {
     this.send('agent:send_message', {
       session_id: sessionId,