feat: global time range context with custom date picker

Lift time range state into a shared React context so the selected
range persists across page navigation and browser refreshes
(localStorage). Add a "Custom" option with a popover date picker
that lets users specify arbitrary start/end dates. All preset end
times now use endOf("day") (23:59:59) instead of the current moment.

app/logs/page.tsx

@@ -4,7 +4,8 @@ import { useEffect, useState, useCallback } from "react";
 import { motion } from "motion/react";
 import { ScrollText, Search, ChevronLeft, ChevronRight, Zap } from "lucide-react";
 import { TimeRangeSelector } from "@/components/TimeRangeSelector";
-import { type TimeRange, getTimeRange, buildQuery, formatNumber, formatTokens, formatDate } from "@/lib/utils";
+import { buildQuery, formatNumber, formatTokens, formatDate } from "@/lib/utils";
+import { useTimeRange } from "@/lib/time-range-context";
 import { useI18n } from "@/lib/i18n";
 
 interface LogEntry {
@@ -16,7 +17,7 @@ interface LogEntry {
 
 export default function LogsPage() {
   const { t } = useI18n();
-  const [range, setRange] = useState<TimeRange>("7d");
+  const { getEffectiveRange } = useTimeRange();
   const [page, setPage] = useState(1);
   const [logs, setLogs] = useState<LogEntry[]>([]);
   const [total, setTotal] = useState(0);
@@ -26,14 +27,14 @@ export default function LogsPage() {
 
   const fetchData = useCallback(async () => {
     setLoading(true);
-    const { start, end } = getTimeRange(range);
+    const { start, end } = getEffectiveRange();
     const res = await fetch(buildQuery("/api/logs", { start, end, page, page_size: pageSize, ...filters }));
     const data = await res.json();
     setLogs(data.logs); setTotal(data.total); setLoading(false);
-  }, [range, page, filters]);
+  }, [page, filters, getEffectiveRange]);
 
   useEffect(() => { fetchData(); }, [fetchData]);
-  useEffect(() => { setPage(1); }, [range, filters]);
+  useEffect(() => { setPage(1); }, [getEffectiveRange, filters]);
   const totalPages = Math.ceil(total / pageSize);
 
   const headers = [t("th.time"), t("th.user"), t("th.realModel"), t("th.channel"), t("th.input"), t("th.output"), t("th.totalToken"), t("th.latency"), ""];
@@ -45,7 +46,7 @@ export default function LogsPage() {
           <ScrollText className="h-5 w-5" style={{ color: "var(--accent)", opacity: 0.6 }} />
           <h1 className="text-2xl font-bold gradient-text">{t("logs.title")}</h1>
         </motion.div>
-        <TimeRangeSelector value={range} onChange={setRange} />
+        <TimeRangeSelector />
       </div>
 
       <motion.div initial={{ opacity: 0, y: 12 }} animate={{ opacity: 1, y: 0 }} className="flex items-center gap-3">