feat(#73): Adaptive Weekend shell and responsive navigation

Implemented by claude via .agents/dev dispatch.

frontend/src/hooks/useWeekendContext.ts

@@ -0,0 +1,148 @@
+import { useEffect, useMemo, useState } from 'react'
+import { useQueries, useQuery } from '@tanstack/react-query'
+import {
+  fetchChampionshipHub,
+  fetchLiveState,
+  fetchLocalMeetings,
+  fetchNews,
+  fetchSeasonMeetings,
+  fetchSeasons,
+  fetchSessions,
+  fetchWeekend,
+  fetchWeekendContext,
+} from '../api'
+import { pickFocusMeeting } from '../lib/schedule'
+import { deriveWeekendContext } from '../lib/weekendContext'
+import type { NewsItem, Weekend, WeekendContext } from '../types'
+
+export interface UseWeekendContextResult {
+  context: WeekendContext
+  /** True when the payload came from the canonical /api/v1/weekend-context endpoint. */
+  fromEndpoint: boolean
+  now: Date
+}
+
+/**
+ * useWeekendContext prefers the canonical /api/v1/weekend-context endpoint and
+ * falls back to deriving the same contract client-side from existing endpoints
+ * so the Weekend home works even when the backend handler (sibling story #72) is
+ * not yet deployed.
+ */
+export function useWeekendContext(): UseWeekendContextResult {
+  const [now, setNow] = useState(() => Date.now())
+  useEffect(() => {
+    const timer = window.setInterval(() => setNow(Date.now()), 1000)
+    return () => window.clearInterval(timer)
+  }, [])
+  const nowDate = useMemo(() => new Date(now), [now])
+
+  const contextQuery = useQuery({
+    queryKey: ['weekend-context'],
+    queryFn: fetchWeekendContext,
+    staleTime: 30_000,
+  })
+
+  const seasonsQuery = useQuery({ queryKey: ['seasons'], queryFn: fetchSeasons })
+  const season = seasonsQuery.data?.[0] ?? null
+
+  const localMeetingsQuery = useQuery({
+    queryKey: ['meetings', season, 'local'],
+    queryFn: () => fetchLocalMeetings(season!),
+    enabled: season != null,
+  })
+
+  const seasonMeetingsQuery = useQuery({
+    queryKey: ['season-meetings', season],
+    queryFn: () => fetchSeasonMeetings(season!),
+    enabled: season != null,
+  })
+
+  const championshipQuery = useQuery({
+    queryKey: ['championship-hub', season],
+    queryFn: () => fetchChampionshipHub(season!),
+    enabled: season != null,
+  })
+
+  const liveQuery = useQuery({ queryKey: ['live-state'], queryFn: fetchLiveState, staleTime: 5_000 })
+
+  const newsQuery = useQuery({ queryKey: ['news', 6], queryFn: () => fetchNews(6) })
+
+  const localMeetings = useMemo(() => localMeetingsQuery.data ?? [], [localMeetingsQuery.data])
+  const seasonMeetings = seasonMeetingsQuery.data?.length ? seasonMeetingsQuery.data : localMeetings
+
+  const weekendQueries = useQueries({
+    queries: localMeetings.map((meeting) => ({
+      queryKey: ['weekend', meeting.meeting_key],
+      queryFn: () => fetchWeekend(meeting.meeting_key),
+      enabled: localMeetings.length > 0,
+      staleTime: 60_000,
+    })),
+  })
+
+  const weekendsByKey = useMemo(() => {
+    const map = new Map<number, Weekend>()
+    localMeetings.forEach((meeting, i) => {
+      const data = weekendQueries[i]?.data
+      if (data) map.set(meeting.meeting_key, data)
+    })
+    return map
+  }, [localMeetings, weekendQueries])
+
+  const focusMeeting = useMemo(() => pickFocusMeeting(seasonMeetings, nowDate), [seasonMeetings, nowDate])
+  const focusHasLocal = focusMeeting ? weekendsByKey.has(focusMeeting.meeting_key) : false
+
+  const focusSessionsQuery = useQuery({
+    queryKey: ['sessions', focusMeeting?.meeting_key, 'openf1'],
+    queryFn: () => fetchSessions(focusMeeting!.meeting_key, 'openf1'),
+    enabled: focusMeeting != null && !focusHasLocal,
+    staleTime: 60_000,
+  })
+
+  const news: NewsItem[] = newsQuery.data ?? []
+
+  const derived = useMemo(
+    () =>
+      deriveWeekendContext({
+        season,
+        meetings: seasonMeetings,
+        weekendsByKey,
+        championship: championshipQuery.data,
+        liveActive: liveQuery.data?.is_live === true,
+        news,
+        focusSessions: focusSessionsQuery.data,
+        now: nowDate,
+      }),
+    [
+      season,
+      seasonMeetings,
+      weekendsByKey,
+      championshipQuery.data,
+      liveQuery.data,
+      news,
+      focusSessionsQuery.data,
+      nowDate,
+    ],
+  )
+
+  if (contextQuery.data) {
+    return { context: contextQuery.data, fromEndpoint: true, now: nowDate }
+  }
+
+  if (seasonsQuery.isLoading || (season != null && seasonMeetingsQuery.isLoading && localMeetingsQuery.isLoading)) {
+    return { context: { state: 'loading', season: season ?? 0 }, fromEndpoint: false, now: nowDate }
+  }
+
+  if (seasonsQuery.isError) {
+    return {
+      context: {
+        state: 'error',
+        season: 0,
+        message: seasonsQuery.error instanceof Error ? seasonsQuery.error.message : 'Failed to load Weekend',
+      },
+      fromEndpoint: false,
+      now: nowDate,
+    }
+  }
+
+  return { context: derived, fromEndpoint: false, now: nowDate }
+}