feat(costs): consolidate /usage into /costs with Spend + Providers tabs

merge Usage page into Costs as two tabs ('Spend' and 'Providers'),
extract shared date-range logic to useDateRange() hook, delete /usage
route and sidebar entry, fix quota-windows bugs from prior review

ui/src/hooks/useDateRange.ts

@@ -0,0 +1,96 @@
+import { useMemo, useState } from "react";
+
+export type DatePreset = "mtd" | "7d" | "30d" | "ytd" | "all" | "custom";
+
+export const PRESET_LABELS: Record<DatePreset, string> = {
+  mtd: "Month to Date",
+  "7d": "Last 7 Days",
+  "30d": "Last 30 Days",
+  ytd: "Year to Date",
+  all: "All Time",
+  custom: "Custom",
+};
+
+export const PRESET_KEYS: DatePreset[] = ["mtd", "7d", "30d", "ytd", "all", "custom"];
+
+function computeRange(preset: DatePreset): { from: string; to: string } {
+  const now = new Date();
+  const to = now.toISOString();
+  switch (preset) {
+    case "mtd": {
+      const d = new Date(now.getFullYear(), now.getMonth(), 1);
+      return { from: d.toISOString(), to };
+    }
+    case "7d": {
+      const d = new Date(now.getTime() - 7 * 24 * 60 * 60 * 1000);
+      return { from: d.toISOString(), to };
+    }
+    case "30d": {
+      const d = new Date(now.getTime() - 30 * 24 * 60 * 60 * 1000);
+      return { from: d.toISOString(), to };
+    }
+    case "ytd": {
+      const d = new Date(now.getFullYear(), 0, 1);
+      return { from: d.toISOString(), to };
+    }
+    case "all":
+    case "custom":
+      return { from: "", to: "" };
+  }
+}
+
+export interface UseDateRangeResult {
+  preset: DatePreset;
+  setPreset: (p: DatePreset) => void;
+  customFrom: string;
+  setCustomFrom: (v: string) => void;
+  customTo: string;
+  setCustomTo: (v: string) => void;
+  /** resolved iso strings ready to pass to api calls; empty string means unbounded */
+  from: string;
+  to: string;
+  /** false when preset=custom but both dates are not yet selected */
+  customReady: boolean;
+}
+
+export function useDateRange(): UseDateRangeResult {
+  const [preset, setPreset] = useState<DatePreset>("mtd");
+  const [customFrom, setCustomFrom] = useState("");
+  const [customTo, setCustomTo] = useState("");
+
+  const { from, to } = useMemo(() => {
+    if (preset === "custom") {
+      // treat custom date strings as local-date boundaries so the full day is included
+      // regardless of the user's timezone. "from" starts at local midnight, "to" at 23:59:59.999.
+      const fromDate = customFrom ? new Date(customFrom + "T00:00:00") : null;
+      const toDate = customTo ? new Date(customTo + "T23:59:59.999") : null;
+      return {
+        from: fromDate ? fromDate.toISOString() : "",
+        to: toDate ? toDate.toISOString() : "",
+      };
+    }
+    const range = computeRange(preset);
+    // floor `to` to the nearest minute so the query key is stable across 30s refetch ticks
+    // (prevents a new cache entry being created on every poll cycle)
+    if (range.to) {
+      const d = new Date(range.to);
+      d.setSeconds(0, 0);
+      range.to = d.toISOString();
+    }
+    return range;
+  }, [preset, customFrom, customTo]);
+
+  const customReady = preset !== "custom" || (!!customFrom && !!customTo);
+
+  return {
+    preset,
+    setPreset,
+    customFrom,
+    setCustomFrom,
+    customTo,
+    setCustomTo,
+    from,
+    to,
+    customReady,
+  };
+}