Bug fixes (real breakage):

Newsletter nudge — was calling /api/study-account/profile (wrong endpoint, ignored subscription). Now correctly calls /api/study-account/preferences with PATCH.
Security:

Email regex — replaced the permissive [^\s@]+@[^\s@]+ pattern with a proper RFC-compliant regex in contact.js and downloads.js
Avatar magic bytes — server now checks actual PNG/JPEG/GIF/WEBP header bytes, not just the data URL prefix
Certificate rate limit — public /api/public/certificate/:token now has a 30 req/15min limiter
Session absolute TTL — admin sessions now have a 30-day hard cap; a stolen token can no longer be kept alive indefinitely by passive reads
Account lockout — 5 failed logins locks a study account for 1 hour
CSP headers — Content-Security-Policy, X-Frame-Options, X-Content-Type-Options, and Referrer-Policy headers added globally
Data integrity:

Cascade delete — deleting a study account now also removes their certificates, community posts, comments, and progress file
UX / reliability:

Escape key on modals — all 3 modal groups (study index, notes, account) now close on Escape
Display name min-length — empty spaces-only names rejected; if provided, must be ≥2 chars
Note save rate limit — 30 saves/minute per user max
Analytics fetch timeout — 5s AbortController so a hanging server doesn't block the browser indefinitely
Email validation on signup — frontend catches bad email formats before hitting the server
Cleanup:

Deduplicated download forms — StudyDownloadForm and ResourceDownloadForm now share a single DownloadForm base; both are now thin wrappers

server/routes/downloads.js

@@ -67,7 +67,7 @@ export function register(app) {
       if (!lastName || typeof lastName !== 'string' || lastName.trim().length < 1 || lastName.trim().length > 100) {
         res.status(400).json({ message: 'Last name is required.' }); return
       }
-      if (!email || typeof email !== 'string' || !/^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email.trim())) {
+      if (!email || typeof email !== 'string' || !/^[a-zA-Z0-9._%+\-]{1,64}@[a-zA-Z0-9.\-]{1,253}\.[a-zA-Z]{2,}$/.test(email.trim())) {
         res.status(400).json({ message: 'A valid email address is required.' }); return
       }
 
@@ -175,7 +175,7 @@ export function register(app) {
       if (!lastName || typeof lastName !== 'string' || lastName.trim().length < 1 || lastName.trim().length > 100) {
         res.status(400).json({ message: 'Last name is required.' }); return
       }
-      if (!email || typeof email !== 'string' || !/^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email.trim())) {
+      if (!email || typeof email !== 'string' || !/^[a-zA-Z0-9._%+\-]{1,64}@[a-zA-Z0-9.\-]{1,253}\.[a-zA-Z]{2,}$/.test(email.trim())) {
         res.status(400).json({ message: 'A valid email address is required.' }); return
       }