add privacy-first analytics with progressive event collection

Three-layer pipeline: Plug for all HTTP requests (no JS needed), LiveView
hook for SPA navigations, JS hook for screen width. ETS-backed buffer
batches writes to SQLite every 10s. Daily-rotating salt for visitor hashing.
Includes admin dashboard with date ranges, visitor trends, top pages,
sources, devices, and e-commerce conversion funnel. Oban cron for 12-month
data retention.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/js/app.js

@@ -509,10 +509,17 @@ const CardRadioScroll = {
   }
 }
 
+// Analytics: send screen width for device classification (Layer 3)
+const AnalyticsInit = {
+  mounted() {
+    this.pushEvent("analytics:screen", { width: window.innerWidth })
+  }
+}
+
 const csrfToken = document.querySelector("meta[name='csrf-token']").getAttribute("content")
 const liveSocket = new LiveSocket("/live", Socket, {
   params: {_csrf_token: csrfToken},
-  hooks: {...colocatedHooks, ColorSync, Lightbox, CartPersist, CartDrawer, ProductImageScroll, SearchModal, CollectionFilters, CardRadioScroll},
+  hooks: {...colocatedHooks, ColorSync, Lightbox, CartPersist, CartDrawer, ProductImageScroll, SearchModal, CollectionFilters, CardRadioScroll, AnalyticsInit},
 })
 
 // Show progress bar on live navigation and form submits