feat(billing): add posthog code token spend analysis banner

Surfaces a self-serve spend analysis inside Settings -> Plan & usage.
Calls the new /api/llm_analytics/posthog_code_spend/ endpoint, renders
totals + breakdowns by ai_product / tool / model + top traces, and
generates inline heuristic suggestions on where to optimise. Footer
links to PostHog LLM analytics docs and the exploring-llm-costs skill.

Gated behind the posthog-code-spend-analysis feature flag (always on
in dev). Plan & usage section itself is also forced on in dev so the
banner is reachable without billing-flag setup.

Generated-By: PostHog Code
Task-Id: f9d5d152-49c6-46cf-8fde-079105ba2e67

Author: pauldambra

apps/code/src/renderer/api/posthogClient.ts

@@ -1,3 +1,4 @@
+import type { SpendAnalysisResponse } from "@features/billing/types/spend-analysis";
 import { isSupportedReasoningEffort } from "@posthog/agent/adapters/reasoning-effort";
 import type { PermissionMode } from "@posthog/agent/execution-mode";
 import {
@@ -2854,4 +2855,24 @@ export class PostHogAPIClient {
     const blob = await response.blob();
     return URL.createObjectURL(blob);
   }
+
+  /** Fetch the requesting user's PostHog Code token spend analysis. */
+  async getPostHogCodeSpendAnalysis(
+    days: number = 30,
+  ): Promise<SpendAnalysisResponse> {
+    const urlPath = `/api/llm_analytics/posthog_code_spend/`;
+    const url = new URL(`${this.api.baseUrl}${urlPath}`);
+    url.searchParams.set("days", String(days));
+    const response = await this.api.fetcher.fetch({
+      method: "get",
+      url,
+      path: urlPath,
+    });
+    if (!response.ok) {
+      throw new Error(
+        `Failed to fetch token spend analysis: ${response.statusText}`,
+      );
+    }
+    return (await response.json()) as SpendAnalysisResponse;
+  }
 }