diff --git a/src/ua.js b/src/ua.js
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+++ b/src/ua.js
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+// ─────────────────────────────────────────────────────────────
+// src/ua.js — minimal User-Agent parser.
+//
+// Coarse-grained classification only: device_type / os / browser.
+// We deliberately do NOT pull in ua-parser-js — the project keeps a
+// small dependency footprint and we only need three buckets. The raw
+// UA is also stored alongside parsed fields (see src/events.js) so a
+// regex miss can be re-classified later.
+//
+// Also owns MOBILE_UA_RE — the existing /timeline view-dispatch
+// regex used by server.js. Single source of truth.
+// ─────────────────────────────────────────────────────────────
+
+// UA substrings that mean "phone-class small screen". Tablets (iPad,
+// Android tablets) deliberately do NOT match — they get the desktop
+// view, which matches existing behaviour in server.js.
+export const MOBILE_UA_RE =
+  /\b(iPhone|iPod|Android.*Mobile|Mobile.*Firefox|IEMobile|BlackBerry|Opera Mini)\b/i;
+
+// Tablet-class devices. Order matters in parseUA(): tablet check runs
+// before mobile so "iPad" doesn't accidentally fall through to desktop.
+const TABLET_UA_RE = /\b(iPad|Android(?!.*Mobile))\b/i;
+
+export function parseUA(ua) {
+  if (!ua || typeof ua !== 'string') {
+    return { device_type: null, os: null, browser: null };
+  }
+
+  // device_type
+  let device_type = 'desktop';
+  if (TABLET_UA_RE.test(ua))      device_type = 'tablet';
+  else if (MOBILE_UA_RE.test(ua)) device_type = 'mobile';
+
+  // os
+  let os = 'other';
+  if      (/\b(iPhone|iPad|iPod)\b/.test(ua))   os = 'iOS';
+  else if (/\bAndroid\b/.test(ua))              os = 'Android';
+  else if (/\bWindows\b/.test(ua))              os = 'Windows';
+  else if (/Mac OS X|Macintosh/.test(ua))       os = 'macOS';
+  else if (/\bLinux\b/.test(ua))                os = 'Linux';
+
+  // browser — order matters. All Chromium UAs include "Safari/", all
+  // Edge UAs include "Chrome/", so the most specific token must win.
+  let browser = 'other';
+  if      (/\bEdg\//.test(ua))     browser = 'Edge';
+  else if (/\bFirefox\//.test(ua)) browser = 'Firefox';
+  else if (/\bChrome\//.test(ua))  browser = 'Chrome';
+  else if (/\bSafari\//.test(ua))  browser = 'Safari';
+
+  return { device_type, os, browser };
+}