feat(quota-share): add StackedAllocationBar component (per-key slices) (B/G4)

New component renders a horizontal stacked bar split by allocation weight,
with optional per-key consumed% labels sourced from PoolUsageSnapshot.dimensions[dimensionIndex].perKey.
Returns null when allocations is empty. Uses shared PALETTE of 8 colors.

src/app/(dashboard)/dashboard/costs/quota-share/components/StackedAllocationBar.tsx

@@ -0,0 +1,115 @@
+"use client";
+
+import { useTranslations } from "next-intl";
+import type { PoolAllocation } from "@/lib/quota/dimensions";
+import type { PoolUsageSnapshot } from "@/lib/quota/types";
+
+export interface StackedAllocationBarProps {
+  allocations: PoolAllocation[];
+  usage: PoolUsageSnapshot | null;
+  keyLabels: Record<string, string>;
+  /** When usage has multiple dimensions, which one to display in this bar.
+   *  Default: the first dimension. */
+  dimensionIndex?: number;
+}
+
+const PALETTE = [
+  "#a78bfa",
+  "#60a5fa",
+  "#34d399",
+  "#fbbf24",
+  "#f87171",
+  "#22d3ee",
+  "#f472b6",
+  "#94a3b8",
+];
+
+export default function StackedAllocationBar({
+  allocations,
+  usage,
+  keyLabels,
+  dimensionIndex = 0,
+}: StackedAllocationBarProps): JSX.Element | null {
+  const t = useTranslations("quotaShare");
+
+  if (allocations.length === 0) {
+    return null;
+  }
+
+  // Build a map of apiKeyId → { consumed, fairShare } from the relevant dimension
+  const perKeyMap: Record<string, { consumed: number; fairShare: number }> = {};
+  if (usage) {
+    const dim = usage.dimensions[dimensionIndex];
+    if (dim) {
+      for (const entry of dim.perKey) {
+        perKeyMap[entry.apiKeyId] = { consumed: entry.consumed, fairShare: entry.fairShare };
+      }
+    }
+  }
+
+  return (
+    <div className="mb-3">
+      <h4 className="text-[10px] uppercase tracking-wide font-bold text-text-muted mb-1.5">
+        {t("stackedBarTitle")}
+      </h4>
+
+      {/* Stacked bar */}
+      <div className="flex h-3 rounded overflow-hidden w-full mb-2">
+        {allocations.map((alloc, i) => {
+          const color = PALETTE[i % PALETTE.length];
+          const keyUsage = perKeyMap[alloc.apiKeyId];
+          let consumedPercent: number | null = null;
+          if (keyUsage && keyUsage.fairShare > 0) {
+            consumedPercent = Math.round((keyUsage.consumed / keyUsage.fairShare) * 100);
+          }
+          const label = keyLabels[alloc.apiKeyId] ?? alloc.apiKeyId;
+          const tooltipText =
+            consumedPercent !== null
+              ? `${label}: ${alloc.weight}% (${t("usedSuffix", { percent: consumedPercent })})`
+              : `${label}: ${alloc.weight}%`;
+          return (
+            <div
+              key={alloc.apiKeyId}
+              style={{ width: `${alloc.weight}%`, backgroundColor: color }}
+              title={tooltipText}
+              aria-label={tooltipText}
+            />
+          );
+        })}
+      </div>
+
+      {/* Labels */}
+      <div className="flex flex-wrap gap-x-3 gap-y-1">
+        {allocations.map((alloc, i) => {
+          const color = PALETTE[i % PALETTE.length];
+          const keyUsage = perKeyMap[alloc.apiKeyId];
+          let consumedPercent: number | null = null;
+          if (keyUsage && keyUsage.fairShare > 0) {
+            consumedPercent = Math.round((keyUsage.consumed / keyUsage.fairShare) * 100);
+          }
+          const label = keyLabels[alloc.apiKeyId] ?? alloc.apiKeyId;
+          return (
+            <span
+              key={alloc.apiKeyId}
+              className="flex items-center gap-1 text-[10px] text-text-muted"
+            >
+              <span
+                className="inline-block w-2 h-2 rounded-sm shrink-0"
+                style={{ backgroundColor: color }}
+              />
+              <span>
+                {label} {alloc.weight}%
+                {consumedPercent !== null && (
+                  <span className="text-text-muted/70">
+                    {" "}
+                    ({t("usedSuffix", { percent: consumedPercent })})
+                  </span>
+                )}
+              </span>
+            </span>
+          );
+        })}
+      </div>
+    </div>
+  );
+}