Mail: account cards, choose which mail apps to show, optional framed webmail

- Mail page redesigned as one card per account (inbox + compose)
- Settings -> Mail apps: tick the apps you use; saved on the server per person
- WEBMAIL_URL shows your own webmail (e.g. Roundcube) inside the Mail tab; provider shortcuts one tap away
- README: Roundcube setup and framing headers

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.env.example

@@ -19,3 +19,6 @@ AUDIOBOOKS_LOGIN_LINK=
 # or per-person signed links (optional): shared secret the audiobooks site uses to verify signed links
 AUDIOBOOKS_SSO_SECRET=
 #AUDIOBOOKS_SSO_URL=https://audiobooks.necloud.us/sso
+
+# Your own webmail to show inside the Mail tab (Roundcube, SnappyMail...). Leave blank to keep the provider shortcuts. See README (Mail).
+WEBMAIL_URL=