fix(sponsor): anchor 14-day toast cooldown on show, not just on CTA click (#2477)

The sponsor toast re-fired on every fresh browser session. The backend
owns the 14-day cooldown but only persists the anchor (last_shown_at) and
the seen-milestone record inside POST /sponsor-prompt/dismiss, and the hook
only called dismiss from the "View supporters" CTA onClick. A user who saw
the toast but never clicked the CTA persisted no state; the per-tab
sessionStorage guard hid the re-fire within one session, but every new
session re-checked against empty state and re-showed the same milestone.

Record the toast as shown the moment it renders (POST /dismiss right after
showPersistentToast) so display is what arms the cooldown. CTA click stays
optional and just navigates. Frontend-only; backend cooldown logic unchanged.

static/index.html

@@ -26,7 +26,7 @@
 
     <!-- Splash screens for iOS -->
     <link rel="apple-touch-startup-image" href="/img/android-chrome-512x512.png" />
-    <script type="module" crossorigin src="/assets/index-DqJZ0C8s.js"></script>
+    <script type="module" crossorigin src="/assets/index-Bp9OAjMR.js"></script>
     <link rel="stylesheet" crossorigin href="/assets/index-BxVhuRti.css">
   </head>
   <body>