[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/state/modelsSlice.ts

@@ -0,0 +1,49 @@
+import { createSlice, createAsyncThunk } from '@reduxjs/toolkit';
+import { API_BASE } from '@/shared/config';
+
+const AGENTS_API = `${API_BASE}/agents`;
+
+export interface ModelOption {
+  value: string;
+  label: string;
+  version?: string;
+  context_window: number;
+}
+
+interface ModelsState {
+  byProvider: Record<string, ModelOption[]>;
+  loaded: boolean;
+}
+
+const initialState: ModelsState = {
+  byProvider: {},
+  loaded: false,
+};
+
+export const fetchModels = createAsyncThunk('models/fetchModels', async () => {
+  const res = await fetch(`${AGENTS_API}/models`);
+  if (!res.ok) throw new Error('Failed to fetch models');
+  const data = await res.json();
+  // API returns { models: { provider: [...] } }
+  const models = data.models || data;
+  return models as Record<string, ModelOption[]>;
+});
+
+const modelsSlice = createSlice({
+  name: 'models',
+  initialState,
+  reducers: {},
+  extraReducers: (builder) => {
+    builder
+      .addCase(fetchModels.fulfilled, (state, action) => {
+        state.byProvider = action.payload;
+        state.loaded = true;
+      })
+      .addCase(fetchModels.rejected, (state) => {
+        // Mark as loaded even on failure so we fall back to hardcoded options
+        state.loaded = true;
+      });
+  },
+});
+
+export default modelsSlice.reducer;