Add the ability to resend unaccepted invites (#60)

Previously the only way to resend was to re-invite the same email from
scratch, creating a whole new invite row. Adds a "Resend" action next
to Revoke on each pending invite -- rotates the token and refreshes the
7-day expiry on the same row (the old link stops working the moment
it's used, same instinct as a password-reset resend), then re-sends
the invite email.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/AdminPage.css

@@ -217,6 +217,14 @@
   cursor: pointer;
 }
 
+.admin-token-resend {
+  background: transparent;
+  border: none;
+  color: var(--ds-accent);
+  font-size: 0.76rem;
+  cursor: pointer;
+}
+
 .admin-issue-token {
   display: flex;
   align-items: center;