feat(mail): rich text editor for signatures + placeholder variables

- Replace textarea in SignatureManager with TipTap RichTextEditor
- Add placeholder variable buttons: {{user.name}}, {{user.first_name}},
  {{user.last_name}}, {{user.email}}, {{user.role}}, {{tenant.name}}, {{date}}
- Add replaceSignatureVariables() utility in mail API
- ComposeModal now replaces variables with actual user/tenant data when inserting signature
- Variables resolved from auth store (user name, email, role, tenant name)

frontend/src/api/mail.ts

@@ -417,6 +417,31 @@ export function decodeMimeHeader(value: string | undefined | null): string {
   }
 }
 
+/**
+ * Replace placeholder variables in signature HTML with actual user/tenant data.
+ * Variables: {{user.name}}, {{user.first_name}}, {{user.last_name}},
+ * {{user.email}}, {{user.role}}, {{tenant.name}}, {{date}}
+ */
+export function replaceSignatureVariables(
+  html: string,
+  user: { name?: string; email?: string; role?: string; first_name?: string; last_name?: string },
+  tenant?: { name?: string },
+): string {
+  const now = new Date();
+  const dateStr = now.toLocaleDateString([], { year: 'numeric', month: '2-digit', day: '2-digit' });
+  const firstName = user.first_name || (user.name ? user.name.split(' ')[0] : '');
+  const lastName = user.last_name || (user.name ? user.name.split(' ').slice(1).join(' ') : '');
+
+  return html
+    .replace(/\{\{user\.name\}\}/gi, user.name || '')
+    .replace(/\{\{user\.first_name\}\}/gi, firstName)
+    .replace(/\{\{user\.last_name\}\}/gi, lastName)
+    .replace(/\{\{user\.email\}\}/gi, user.email || '')
+    .replace(/\{\{user\.role\}\}/gi, user.role || '')
+    .replace(/\{\{tenant\.name\}\}/gi, tenant?.name || '')
+    .replace(/\{\{date\}\}/gi, dateStr);
+}
+
 // ─── Folders ────────────────────────────────────────────────────────────────
 
 export function fetchFolders(accountId: string): Promise<MailFolder[]> {