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/index.html

@@ -826,12 +826,15 @@
           kAutoChainOwn: "独立链",
           kAutoChainGlobal: "跟随全局",
           kAutoChainTitle: "独立 AUTO 链",
-          kAutoChainHint: "为空则跟随全局 AUTO 链",
+          kAutoChainHint: "默认复制全局链供你增删；清空并保存即恢复跟随全局",
           kAutoChainInherit: "使用全局链",
           kAutoChainAdd: "+ 添加槽位",
           kAutoChainSave: "保存",
           kAutoChainTier: "层级",
           kAutoChainEmpty: "该密钥使用全局 AUTO 链",
+          kAutoChainSeeded: "已复制全局 AUTO 链作为编辑起点，保存后该密钥将使用这条独立链",
+          kAutoChainNewOwn: "该密钥当前跟随全局链，保存后改用这里配置的独立链",
+          kAutoChainWillInherit: "保存后该密钥将恢复跟随全局 AUTO 链",
           kAutoChainBad: "已保存，但槽位无法解析（模型与源的组合不存在），AUTO 请求会失败",
           kName: "名称",
           kRole: "角色",
@@ -1097,12 +1100,15 @@
           kAutoChainOwn: "own",
           kAutoChainGlobal: "global",
           kAutoChainTitle: "Per-key AUTO chain",
-          kAutoChainHint: "Leave empty to inherit the global AUTO chain",
+          kAutoChainHint: "Starts from a copy of the global chain; clear it and save to follow the global chain again",
           kAutoChainInherit: "Use global chain",
           kAutoChainAdd: "+ Add slot",
           kAutoChainSave: "Save",
           kAutoChainTier: "Tier",
           kAutoChainEmpty: "This key uses the global AUTO chain",
+          kAutoChainSeeded: "Copied the global AUTO chain as a starting point — saving gives this key its own chain",
+          kAutoChainNewOwn: "This key currently follows the global chain; saving switches it to the chain configured here",
+          kAutoChainWillInherit: "Saving makes this key follow the global AUTO chain again",
           kAutoChainBad: "Saved, but the slot does not resolve (no source serves that model); AUTO requests will fail",
           kName: "Name",
           kRole: "Role",
@@ -5272,48 +5278,66 @@ function afterChainEdit() {
         // Park the key on the modal so a click that bubbles up cannot hit the
         // global editor behind it.
         wrap.addEventListener("click", (ev) => ev.stopPropagation());
-        try {
-          const j = await api("/api/v1/sources");
-          const idx = buildSortIndex(j.sources || []);
-          const cur = await api("/api/keys/" + encodeURIComponent(key) + "/auto");
-          keyAutoState.key = key;
-          keyAutoState.inherits = !!cur.inherits;
-          // A key with no own chain shows an EMPTY canvas, never the discovery
-          // fallback buildLanes uses for an unconfigured global chain: that
-          // fallback fills every source's models (226 blocks on a real
-          // deployment), and showing them inside "this key's chain" dialog
-          // says they already belong to this user. They do not.
-          sortState.scope = key;
-          sortState.kind = "chat";
-          sortState.lanes =
-            cur.auto && cur.auto.length ? buildLanes(idx, cur.auto, "chat") : [];
-          sortState.origin = JSON.stringify(sortState.lanes);
-          await loadModelPairs();
-          renderSortEditor($("#ka-canvas-host"), { canvasId: "key-auto-canvas" });
-          // The notice is the only thing telling the user what an empty
-          // canvas means here. Without it the dialog is just a blank box and
-          // "Use global chain" looks like a button that already did
-          // something. It disappears as soon as the chain has slots of its
-          // own, which keyAutoSave's reload then confirms.
-          if (keyAutoState.inherits) keyAutoInheritNotice();
-        } catch (e) {
-          toast(String(e));
-        }
+try {
+            const j = await api("/api/v1/sources");
+            const idx = buildSortIndex(j.sources || []);
+            const cur = await api("/api/keys/" + encodeURIComponent(key) + "/auto");
+            keyAutoState.key = key;
+            keyAutoState.inherits = !!cur.inherits;
+            // A key with no chain of its own starts from a COPY OF THE GLOBAL
+            // CHAIN, not from an empty canvas. The realistic task is "give this
+            // user the global chain minus the two sources they must not hit",
+            // and starting blank forces the admin to retype every tier from a
+            // list of 226 models. This is NOT the discovery fallback (which
+            // dumps every model of every source): it is the same 10 rules the
+            // global chain actually runs.
+            let seededFromGlobal = false;
+            let rules = cur.auto || [];
+            if (!rules.length) {
+              try {
+                rules = ((await api("/api/auto")).rules) || [];
+                seededFromGlobal = rules.length > 0;
+              } catch (e) {
+                rules = [];
+              }
+            }
+            sortState.scope = key;
+            sortState.kind = "chat";
+            sortState.lanes = buildLanes(idx, rules, "chat");
+            sortState.origin = JSON.stringify(sortState.lanes);
+            await loadModelPairs();
+            renderSortEditor($("#ka-canvas-host"), { canvasId: "key-auto-canvas" });
+            keyAutoState.seeded = seededFromGlobal;
+            keyAutoInheritNotice();
+          } catch (e) {
+            toast(String(e));
+          }
       }
-      const keyAutoState = { key: null, inherits: true };
-      // The notice describes what an EMPTY canvas means, so it must track the
-      // canvas rather than the state the dialog opened with: adding the first
-      // slot makes "this key uses the global chain" false, and leaving it up
-      // reads as a contradiction right next to the slot the user just added.
-      function keyAutoInheritNotice(showing) {
+      const keyAutoState = { key: null, inherits: true, seeded: false };
+      // The notice states what the canvas content MEANS relative to the server, which
+      // is the one thing the dialog cannot show visually: a full canvas can be
+      // either "this key's own chain (already saved)" or "a copy of the global
+      // chain that only becomes this key's chain on Save". Those two look
+      // identical and behave very differently if the admin hits Cancel.
+      function keyAutoInheritNotice() {
         const host = $("#ka-canvas-host");
         if (!host) return;
         let n = host.querySelector(".ka-inherit-note");
         const hasSlots = (sortState.lanes || []).some(
           (l) => (l.models || []).length,
         );
-        const want = showing === undefined ? !hasSlots : showing && !hasSlots;
-        if (!want) {
+        let msg = "";
+        if (!hasSlots) {
+          // Empty canvas: saving restores inheritance.
+          msg = keyAutoState.inherits
+            ? t("kAutoChainEmpty")
+            : t("kAutoChainWillInherit");
+        } else if (keyAutoState.inherits && keyAutoState.seeded) {
+          msg = t("kAutoChainSeeded");
+        } else if (keyAutoState.inherits) {
+          msg = t("kAutoChainNewOwn");
+        }
+        if (!msg) {
           if (n) n.remove();
           return;
         }
@@ -5323,12 +5347,13 @@ function afterChainEdit() {
           n.style.margin = "0 0 10px";
           host.appendChild(n);
         }
-        n.textContent = t("kAutoChainEmpty");
+        n.textContent = msg;
       }
       // "Use the global chain" empties the lanes; saving then PUTs an empty
       // list, which the server treats as clearing the override.
       function keyAutoInherit() {
         keyAutoState.inherits = true;
+        keyAutoState.seeded = false;
         sortState.lanes = [];
         sortState.origin = "[]";
         const host = $("#ka-canvas-host");
@@ -5345,6 +5370,7 @@ function afterChainEdit() {
         // Leaving scope set would make the global editor save to this key.
         sortState.scope = null;
         keyAutoState.key = null;
+        keyAutoState.seeded = false;
       }
       async function keyAutoSave(btn) {
         const key = keyAutoState.key;