feat(ui): per-key AUTO 链默认复制全局链作为编辑起点

此前给没有独立链的密钥打开编辑器是空白画布，管理员要从 226 个模型里
一条条重新挑。真实任务是"这个用户拿全局链，但去掉他不该碰的两个源"，
所以起点应该是全局链本身。

- 无独立链时读 /api/auto 的 rules 预填（实测 4 泳道 / 10 块，与全局链
  一致），不是 discovery 兜底（那会把每个源的每个模型都堆进来）。
- 提示区分三种状态，因为它们在画布上长得一样、点取消的后果完全不同：
  复制来的起点 / 该密钥已有的独立链 / 清空后保存将恢复跟随全局。
- seeded 标记随关闭清理，避免下次的提示说错来源。

CDP 实测：打开无独立链的密钥 → 画布 4 泳道 10 块 + 提示"已复制全局链
作为编辑起点"；删一个块服务端仍 inherits:true；保存后 inherits:false
n=9、弹窗关闭；重开读回 4 泳道 9 块且提示消失（它现在有自己的链了）。
横向滚动可达性一并验证：scrollLeft 172 = scrollWidth-clientWidth，
滚到底后无残留不可达块。

判据 2 条 + 变异（恢复空白起点 / 去掉 seeded 标记均判红）。

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

internal/gateway/ui_key_auto_test.go

@@ -287,3 +287,45 @@ func TestUIStatsPeriodRangeHandlesHourlyBuckets(t *testing.T) {
 			"this throws on the day view and leaves the range line empty")
 	}
 }
+
+// A key with no chain of its own must open on a COPY OF THE GLOBAL CHAIN, not
+// on an empty canvas. The realistic admin task is "this user gets the global chain
+// minus the sources they must not hit"; starting blank forces retyping every
+// tier against a 226-model picker. This is deliberately NOT the discovery
+// fallback (which dumps every model of every source) — it is the same rules
+// the global chain actually runs.
+func TestUIKeyAutoSeedsFromGlobalChain(t *testing.T) {
+	src := uiSource(t)
+	body := readFuncBody(t, src, "openKeyAutoModal")
+	if !strings.Contains(body, `api("/api/auto")`) {
+		t.Fatal("openKeyAutoModal must read the global chain to seed the editor")
+	}
+	if !strings.Contains(body, "seededFromGlobal") {
+		t.Fatal("the editor must remember whether its content came from the " +
+			"global chain, so the notice can say so")
+	}
+	// And it must not fall back to the discovery path that fills 226 models.
+	if strings.Contains(body, `buildLanes(idx, [], `) {
+		t.Fatal("seeding from an empty rule list triggers buildLanes discovery")
+	}
+}
+
+// The notice must distinguish "already this key's chain" from "a copy that
+// only becomes its chain on Save" — those two look identical on the canvas and
+// behave very differently if the admin hits Cancel.
+func TestUIKeyAutoNoticeDistinguishesSavedFromCopied(t *testing.T) {
+	src := uiSource(t)
+	body := readFuncBody(t, src, "keyAutoInheritNotice")
+	for _, k := range []string{"kAutoChainSeeded", "kAutoChainNewOwn", "kAutoChainWillInherit"} {
+		if !strings.Contains(body, k) {
+			t.Fatalf("the notice never uses %s — a copied chain and a saved one "+
+				"would look the same", k)
+		}
+	}
+	// Both languages, or a non-Chinese admin sees raw keys.
+	for _, k := range []string{"kAutoChainSeeded", "kAutoChainNewOwn", "kAutoChainWillInherit"} {
+		if n := strings.Count(src, k+`: "`); n < 2 {
+			t.Fatalf("i18n key %s defined %d time(s), want both zh and en", k, n)
+		}
+	}
+}