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/pages/DashboardPage.tsx

@@ -1,5 +1,5 @@
 import React, { useEffect, useState, useRef } from 'react'
-import { useNavigate } from 'react-router-dom'
+import { useNavigate, useSearchParams } from 'react-router-dom'
 import { tripsApi } from '../api/client'
 import { tripRepo } from '../repo/tripRepo'
 import { useAuthStore } from '../store/authStore'
@@ -689,6 +689,7 @@ export default function DashboardPage(): React.ReactElement {
   }
 
   const navigate = useNavigate()
+  const [searchParams, setSearchParams] = useSearchParams()
   const toast = useToast()
   const { t, locale } = useTranslation()
   const { demoMode, user } = useAuthStore()
@@ -709,6 +710,13 @@ export default function DashboardPage(): React.ReactElement {
     return () => { document.body.style.overflow = '' }
   }, [showWidgetSettings])
 
+  useEffect(() => {
+    if (searchParams.get('create') === '1') {
+      setShowForm(true)
+      setSearchParams({}, { replace: true })
+    }
+  }, [searchParams])
+
   useEffect(() => { loadTrips() }, [])
 
   const loadTrips = async () => {

client/src/pages/Trips/noticeActions.ts

@@ -0,0 +1,7 @@
+import { registerNoticeAction } from '../../components/SystemNotices/noticeActions.js';
+
+// Opens the new-trip creation modal on DashboardPage via URL param.
+// DashboardPage reads ?create=1 on mount and calls setShowForm(true).
+registerNoticeAction('open:trip-create', ({ navigate }) => {
+  navigate('/dashboard?create=1');
+});