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_key_auto_test.go

@@ -266,3 +266,24 @@ func TestUIKeyAutoEditsDoNotAutoPersist(t *testing.T) {
 		}
 	}
 }
+
+// The stats range line formats its end date from the LAST bucket label.
+// Bucket labels are hourly for the day view ("2026-10-03T09") and daily for
+// week/month ("2026-10-01"), so blindly appending "T00:00:00Z" to the hourly
+// form builds "2026-10-03T09T00:00:00Z" — Date rejects it, toISOString()
+// throws, and the assignment is lost. The symptom is an empty range line on
+// exactly the view people look at most, with no error anywhere.
+func TestUIStatsPeriodRangeHandlesHourlyBuckets(t *testing.T) {
+	src := uiSource(t)
+	el := readFuncBody(t, src, "paintStats")
+	// It must branch on the label shape instead of concatenating blindly.
+	if !strings.Contains(el, `T\d{2}$`) && !strings.Contains(el, `\d{2}-\d{2}T\d{2}`) {
+		t.Fatal("paintStats must recognise the hourly bucket label shape " +
+			"(YYYY-MM-DDTHH); concatenating a time onto it yields an invalid Date")
+	}
+	// And it must not contain the naive form that broke it.
+	if strings.Contains(el, `bucket + "T00:00:00Z"`) {
+		t.Fatal("paintStats still appends T00:00:00Z to the bucket label — " +
+			"this throws on the day view and leaves the range line empty")
+	}
+}