Add invite link to Admin panel

The Admin panel now shows the crew's join link (origin/#/game/{id}/join)
with Open and Copy buttons, so an admin can invite new players without
returning to the lobby. Matches the existing rejoin-link copy pattern.

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

frontend/src/pages/Admin.svelte

@@ -10,11 +10,17 @@
     let error = '';
     let copiedPlayerId = null;
     let copiedTimer = null;
+    let copiedInvite = false;
+    let copiedInviteTimer = null;
 
     function rejoinLink(playerId) {
         return `${window.location.origin}/#/game/${gameId}/player/${playerId}`;
     }
 
+    function inviteLink() {
+        return `${window.location.origin}/#/game/${gameId}/join`;
+    }
+
     function copyRejoinLink(playerId) {
         navigator.clipboard.writeText(rejoinLink(playerId));
         copiedPlayerId = playerId;
@@ -22,6 +28,13 @@
         copiedTimer = setTimeout(() => { copiedPlayerId = null; }, 2000);
     }
 
+    function copyInviteLink() {
+        navigator.clipboard.writeText(inviteLink());
+        copiedInvite = true;
+        clearTimeout(copiedInviteTimer);
+        copiedInviteTimer = setTimeout(() => { copiedInvite = false; }, 2000);
+    }
+
     onMount(() => {
         // Try to load admin_key from local storage
         adminKey = localStorage.getItem(`admin_key_${gameId}`) || '';
@@ -75,6 +88,16 @@
             <p><strong>Phase:</strong> {data.game.phase}</p>
             <p><strong>Admin Key:</strong> <span class="admin-key-chip">{data.game.admin_key}</span></p>
 
+            <h3 class="mt-8 mb-4">Invite Link</h3>
+            <p class="info-text">Share this link to let new players join the crew.</p>
+            <div class="link-copy-action">
+                <input type="text" readonly value={inviteLink()} class="input-field flex-grow" />
+                <a href={inviteLink()} class="btn btn-secondary btn-small">Open</a>
+                <button on:click={copyInviteLink} class="btn btn-secondary btn-small">
+                    {copiedInvite ? '✓ Copied' : 'Copy'}
+                </button>
+            </div>
+
             <h3 class="mt-8 mb-4">Players</h3>
             <table class="admin-table">
                 <thead>