feat(system-notices): add personal thank-you notice for v3.0.0

Personal note from the creator shown as the first page in the 3.0
upgrade modal. Includes community links (Discord, Ko-fi) and a
special shout-out to jubnl. Modal UX improved: users must click
through all pages before dismissing, wider layout, enhanced
markdown rendering with styled links, signature, and HR separator.
i18n coverage across all 15 languages.

server/src/systemNotices/registry.ts

@@ -100,6 +100,20 @@ export const SYSTEM_NOTICES: SystemNotice[] = [
     priority: 70,
   },
 
+  {
+    // Page 1 — personal thank-you from the creator (shown first)
+    id: 'v3-thankyou',
+    display: 'modal',
+    severity: 'info',
+    icon: 'Heart',
+    titleKey: 'system_notice.v3_thankyou.title',
+    bodyKey:  'system_notice.v3_thankyou.body',
+    dismissible: true,
+    conditions: [{ kind: 'existingUserBeforeVersion', version: '3.0.0' }],
+    publishedAt: '2026-04-16T00:00:00Z',
+    priority: 95,
+  },
+
   // ── Onboarding ─────────────────────────────────────────────────────────────
 
   {