feat(calendar): implement local calendar invitations with email notifications

- Added functionality to create, accept, decline, and delete local calendar invitations.
- Integrated email notifications for calendar event invitations and deletions.
- Updated database schema to support local invitations and outbound event tracking.
- Enhanced the calendar UI to display pending invitations and allow users to manage them.
- Localized new strings for invitations in English and German.

server/config/mailer.js

@@ -116,7 +116,7 @@ export async function sendFederationInviteEmail(to, name, fromUser, roomName, me
   await transporter.sendMail({
     from: `"${headerAppName}" <${from}>`,
     to,
-    subject: `${headerAppName} – Meeting invitation from ${sanitizeHeaderValue(fromUser)}`,
+    subject: `${headerAppName} - Meeting invitation from ${sanitizeHeaderValue(fromUser)}`,
     html: `
       <div style="font-family:Arial,sans-serif;max-width:520px;margin:0 auto;padding:32px;background:#1e1e2e;color:#cdd6f4;border-radius:12px;">
         <h2 style="color:#cba6f7;margin-top:0;">Hey ${safeName} 👋</h2>
@@ -140,6 +140,83 @@ export async function sendFederationInviteEmail(to, name, fromUser, roomName, me
   });
 }
 
+/**
+ * Send a calendar event invitation email (federated).
+ */
+export async function sendCalendarInviteEmail(to, name, fromUser, title, startTime, endTime, description, inboxUrl, appName = 'Redlight') {
+  if (!transporter) return;
+
+  const from = process.env.SMTP_FROM || process.env.SMTP_USER;
+  const headerAppName = sanitizeHeaderValue(appName);
+  const safeName = escapeHtml(name);
+  const safeFromUser = escapeHtml(fromUser);
+  const safeTitle = escapeHtml(title);
+  const safeDesc = description ? escapeHtml(description) : null;
+
+  const formatDate = (iso) => {
+    try { return new Date(iso).toLocaleString('en-GB', { dateStyle: 'full', timeStyle: 'short' }); }
+    catch { return iso; }
+  };
+
+  await transporter.sendMail({
+    from: `"${headerAppName}" <${from}>`,
+    to,
+    subject: `${headerAppName} - Calendar invitation from ${sanitizeHeaderValue(fromUser)}`,
+    html: `
+      <div style="font-family:Arial,sans-serif;max-width:520px;margin:0 auto;padding:32px;background:#1e1e2e;color:#cdd6f4;border-radius:12px;">
+        <h2 style="color:#cba6f7;margin-top:0;">Hey ${safeName} 👋</h2>
+        <p>You have received a calendar invitation from <strong style="color:#cdd6f4;">${safeFromUser}</strong>.</p>
+        <div style="background:#313244;border-radius:8px;padding:16px;margin:20px 0;">
+          <p style="margin:0 0 4px 0;font-size:15px;font-weight:bold;color:#cdd6f4;">${safeTitle}</p>
+          <p style="margin:6px 0 0 0;font-size:13px;color:#a6adc8;">🕐 ${escapeHtml(formatDate(startTime))} – ${escapeHtml(formatDate(endTime))}</p>
+          ${safeDesc ? `<p style="margin:10px 0 0 0;font-size:13px;color:#a6adc8;font-style:italic;">&quot;${safeDesc}&quot;</p>` : ''}
+        </div>
+        <p style="text-align:center;margin:28px 0;">
+          <a href="${inboxUrl}"
+             style="display:inline-block;background:#cba6f7;color:#1e1e2e;padding:12px 32px;border-radius:8px;text-decoration:none;font-weight:bold;">
+            View Invitation
+          </a>
+        </p>
+        <hr style="border:none;border-top:1px solid #313244;margin:24px 0;"/>
+        <p style="font-size:12px;color:#585b70;">Open the link above to accept or decline the invitation.</p>
+      </div>
+    `,
+    text: `Hey ${name},\n\nYou have received a calendar invitation from ${fromUser}.\nEvent: ${title}\nTime: ${formatDate(startTime)} \u2013 ${formatDate(endTime)}${description ? `\n\n"${description}"` : ''}\n\nView invitation: ${inboxUrl}\n\n\u2013 ${appName}`,
+  });
+}
+
+/**
+ * Notify a user that a federated calendar event they received was deleted by the organiser.
+ */
+export async function sendCalendarEventDeletedEmail(to, name, fromUser, title, appName = 'Redlight') {
+  if (!transporter) return;
+
+  const from = process.env.SMTP_FROM || process.env.SMTP_USER;
+  const headerAppName = sanitizeHeaderValue(appName);
+  const safeName = escapeHtml(name);
+  const safeFromUser = escapeHtml(fromUser);
+  const safeTitle = escapeHtml(title);
+
+  await transporter.sendMail({
+    from: `"${headerAppName}" <${from}>`,
+    to,
+    subject: `${headerAppName} – Calendar event cancelled: ${sanitizeHeaderValue(title)}`,
+    html: `
+      <div style="font-family:Arial,sans-serif;max-width:520px;margin:0 auto;padding:32px;background:#1e1e2e;color:#cdd6f4;border-radius:12px;">
+        <h2 style="color:#f38ba8;margin-top:0;">Hey ${safeName} 👋</h2>
+        <p>The following calendar event was deleted by the organiser (<strong style="color:#cdd6f4;">${safeFromUser}</strong>) and is no longer available:</p>
+        <div style="background:#313244;border-radius:8px;padding:16px;margin:20px 0;border-left:4px solid #f38ba8;">
+          <p style="margin:0;font-size:15px;font-weight:bold;color:#cdd6f4;">${safeTitle}</p>
+        </div>
+        <p style="font-size:13px;color:#7f849c;">The event has been automatically removed from your calendar.</p>
+        <hr style="border:none;border-top:1px solid #313244;margin:24px 0;"/>
+        <p style="font-size:12px;color:#585b70;">This message was sent automatically by ${escapeHtml(appName)}.</p>
+      </div>
+    `,
+    text: `Hey ${name},\n\nThe calendar event "${title}" by ${fromUser} has been deleted and removed from your calendar.\n\n\u2013 ${appName}`,
+  });
+}
+
 /**
  * Send a user registration invite email.
  * @param {string} to          – recipient email