fix(ui): 统计区间文案在日视图恒为空、天数按小时桶误报

v1.9.0 加的「统计区间：起 → 止 (Nd)」正是用来解释"周 > 月"的，但它自己
在日视图上从来不出现在屏幕上。

原因：区间终点从最后一个 bucket 的标签拼出来，而 bucket 粒度随视图变化
—— 日视图按小时（"2026-10-03T09"），周/月按天（"2026-10-01"）。代码无条件
拼 "T00:00:00Z"，日视图就成了 "2026-10-03T09T00:00:00Z"，Date 拒绝该值，
toISOString() 抛 RangeError，整个赋值语句丢失。页面无任何报错，只是那行
字不见了——恰好是用户最常看的"今日"视图。

第二个问题同源：天数直接用了 st.buckets.length。日视图 5 个桶是 5 个小时，
界面却显示 "(5d)"。改为按窗口起止算天数。

CDP 实测（修复后）：
  day   统计区间： 2026-10-03 → 2026-10-03 (1d)   ← 原为空，且曾误报 5d
  week  统计区间： 2026-09-28 → 2026-10-03 (6d)
  month 统计区间： 2026-10-01 → 2026-10-03 (3d)
  all  （空，正确：终身累计没有窗口）

判据一条 + 变异：恢复原来的盲目拼接 → 判红。

Co-Authored-By: ModelRouter <noreply@modelrouter.dev>

internal/gateway/ui/index.html

@@ -2016,16 +2016,41 @@ const pnote = $("#period-note");
           // like a bug unless the dates are on screen — so print them.
           const prange = $("#period-range");
           if (prange && st.period && st.period !== "all" && st.from) {
-            const d0 = new Date(st.from);
-            const d1 = st.buckets && st.buckets.length
-              ? new Date(st.buckets[st.buckets.length - 1].bucket + "T00:00:00Z")
-              : d0;
-            const f = (d) =>
-              d.toISOString().slice(0, 10);
-            prange.textContent =
-              t("periodRange") +
-              " " + f(d0) + " → " + f(d1) +
-              (st.buckets ? " (" + st.buckets.length + "d)" : "");
+            // Bucket labels are hourly for a day ("2026-10-03T09") and daily
+            // for a week or month ("2026-10-01"). Appending "T00:00:00Z" to
+            // the hourly form builds "2026-10-03T09T00:00:00Z", which Date
+            // rejects — toISOString() then throws and the whole assignment is
+            // lost, leaving the range line silently empty on the day view (the
+            // one people look at most).
+            const f = (d) => d.toISOString().slice(0, 10);
+            const label = st.buckets && st.buckets.length
+              ? st.buckets[st.buckets.length - 1].bucket
+              : "";
+            let endMs = 0;
+            if (/^\d{4}-\d{2}-\d{2}$/.test(label)) {
+              endMs = new Date(label + "T00:00:00Z").getTime();
+            } else if (/^\d{4}-\d{2}-\d{2}T\d{2}$/.test(label)) {
+              // An hourly bucket covers its hour, so the day it belongs to is
+              // what the range should end on.
+              endMs = new Date(label + ":00:00Z").getTime();
+            }
+            const from = new Date(st.from);
+            const end = endMs ? new Date(endMs) : from;
+            // The bucket count is not a day count: the day view buckets by
+            // HOUR, so "(5d)" for five hours of today was simply wrong. Days
+            // covered comes from the window itself, which is what the reader
+            // is trying to establish.
+            let days = 1;
+            if (endMs) {
+              days = Math.max(
+                1,
+                Math.round((endMs - from.getTime()) / 86400000) + 1,
+              );
+            }
+            const tail = " (" + days + "d)";
+            prange.textContent = endMs
+              ? t("periodRange") + " " + f(from) + " → " + f(end) + tail
+              : t("periodRange") + " " + f(from) + tail;
           } else if (prange) {
             prange.textContent = "";
           }