fix: defer PostHog/Sentry loading until user consents to telemetry

PostHog SDK was initialized on app mount based only on the server-level
config flag, ignoring user consent. This caused network requests to
us-assets.i.posthog.com (config.js, web-vitals.js, dead-clicks-autocapture.js)
even when the user had not opted in or had explicitly declined telemetry.

- Replace static imports of posthog-js and @sentry/react with dynamic
  import() so the SDK bundles are not downloaded until consent is granted
- Gate initAnalytics on analyticsConsent.analyticsEnabled === true,
  not just server config.enabled
- Add consent re-check after each await import() to handle revocation
  during the async load
- Add shutdownAnalytics() that calls opt_out_capturing() + reset()
  for mid-session consent revocation
- setAnalyticsConsent(false) now triggers full SDK shutdown automatically
- Rewrite analytics test suite with 44 tests covering init gating,
  shutdown lifecycle, consent toggle, race conditions, and Sentry callbacks

Closes #98

apps/web/src/lib/analytics.ts

@@ -1,8 +1,8 @@
-import * as Sentry from "@sentry/react";
 import type { AnalyticsConfig } from "@snapotter/shared";
-import posthogJs from "posthog-js";
 
-let posthog: import("posthog-js").PostHog | null = null;
+type PostHogInstance = import("posthog-js").PostHog;
+
+let posthog: PostHogInstance | null = null;
 let initialized = false;
 let consentGranted = false;
 
@@ -14,11 +14,16 @@ function scrubString(str: string): string {
   return str.replace(FILE_EXT_PATTERN, ".[REDACTED]").replace(FILE_PATH_PATTERN, "/[REDACTED]/");
 }
 
-export function initAnalytics(config: AnalyticsConfig): void {
+export async function initAnalytics(config: AnalyticsConfig): Promise<void> {
   if (initialized || !config.enabled) return;
   initialized = true;
 
   try {
+    const posthogJs = (await import("posthog-js")).default;
+    if (!consentGranted) {
+      initialized = false;
+      return;
+    }
     posthog =
       posthogJs.init(config.posthogApiKey, {
         api_host: config.posthogHost,
@@ -35,11 +40,16 @@ export function initAnalytics(config: AnalyticsConfig): void {
         persistence: "localStorage",
       }) ?? null;
   } catch {
-    // SDK blocked or unavailable — use null provider
+    // SDK blocked or unavailable
   }
 
   try {
     if (config.sentryDsn) {
+      const Sentry = await import("@sentry/react");
+      if (!consentGranted) {
+        initialized = false;
+        return;
+      }
       Sentry.init({
         dsn: config.sentryDsn,
         sendDefaultPii: false,
@@ -81,8 +91,25 @@ export function initAnalytics(config: AnalyticsConfig): void {
   }
 }
 
+export function shutdownAnalytics(): void {
+  if (posthog) {
+    try {
+      posthog.opt_out_capturing();
+      posthog.reset();
+    } catch {
+      // never throw
+    }
+  }
+  posthog = null;
+  initialized = false;
+  consentGranted = false;
+}
+
 export function setAnalyticsConsent(enabled: boolean): void {
   consentGranted = enabled;
+  if (!enabled) {
+    shutdownAnalytics();
+  }
 }
 
 export function identify(instanceId: string, properties: Record<string, unknown>): void {