feat(notices): add system notice infrastructure

Server-side notice registry with per-user condition evaluation (firstLogin,
existingUserBeforeVersion, addonEnabled, dateWindow, role, custom).
Notices are sorted by priority then severity, filtered against dismissals
stored in a new user_notice_dismissals table, and served via
GET /api/system-notices/active + POST /api/system-notices/:id/dismiss.

Client renders notices through a host component that partitions by
display type (modal / banner / toast). The modal renderer supports
multi-page pagination with directional slide transitions, keyboard
navigation, and correct dismiss-all semantics on CTA / X / ESC.
Dismissals are optimistic with a single background retry.

Includes 3.0.0 upgrade notices (v3-photos, v3-journey, v3-features),
onboarding welcome modal, and full i18n coverage across 15 languages.
The /journey route is addon-gated on both client and server.

Also includes: unit + integration test suites, registry integrity test
that validates action CTA IDs against client source, and technical
documentation in docs/system-notices.md.

client/src/components/SystemNotices/SystemNoticeHost.tsx

@@ -0,0 +1,31 @@
+import React, { useEffect } from 'react';
+import { useSystemNoticeStore } from '../../store/systemNoticeStore.js';
+import { ModalRenderer } from './SystemNoticeModal.js';
+import { BannerRenderer, ToastRenderer } from './SystemNoticeBanner.js';
+
+export function SystemNoticeHost() {
+  const { notices, loaded } = useSystemNoticeStore();
+
+  // Notices are fetched by authStore after login (see App.tsx / authStore modification).
+  // Cold-session fetch (page reload with valid session) is triggered here:
+  useEffect(() => {
+    // Only fetch if not already loaded (authStore may have already triggered)
+    if (!loaded) {
+      useSystemNoticeStore.getState().fetch();
+    }
+  }, []);  // eslint-disable-line react-hooks/exhaustive-deps
+
+  if (!loaded) return null;
+
+  const modals  = notices.filter(n => n.display === 'modal');
+  const banners = notices.filter(n => n.display === 'banner');
+  const toasts  = notices.filter(n => n.display === 'toast');
+
+  return (
+    <>
+      <BannerRenderer notices={banners} />
+      <ModalRenderer  notices={modals}  />
+      <ToastRenderer  notices={toasts}  />
+    </>
+  );
+}