fix(ui): per-key 编辑器画布画错容器、保存成功却不关弹窗

CDP 实测发现两个真 bug，都是复用画布时引入的。

一、画布画进了后台标签页。sortCanvasEl() 原本按文档顺序找，先命中
#scr-canvas（全局优先级页的画布）。而弹窗是挂在设置页之上的，两个画布
同时存在于 DOM —— 于是 per-key 的 36 条泳道被画进隐藏的全局画布，弹窗
里空空如也，回���全局页还会发现自己的链被覆盖了。

改为按 sortState.scope 解析：有 scope 就用 #key-auto-canvas，否则用
#scr-canvas。

二、保存成功却弹窗不关。!j.slots 分支里我写了 return，跳过了
keyAutoClose。写确实成功了（服务端已存），但用户看到弹窗还开着、按钮
还是禁用状态，会反复点保存。而且那条 toast 文案写成"保存后 AUTO 请求
会失败"，读起来像保存失败了 —— 一并改成"已保存，但槽位无法解析…"。

判据两条，变异验证：恢复按文档顺序查找 → 第一条判红；!slots 分支加
return → 第二条判红。第二条最初只查 keyAutoClose() 是否存在，变异加了
return 仍然通过，属于判据漏放，已改为扫描 try 块内 close 调用之前的
所有 return 语句。

CDP 复测（先开全局页制造干扰，再开 per-key 弹窗）：
- 弹窗画布 36 泳道，全局画布保持 4 泳道未被污染
- 保存 gpt-5.2@xinjianya 后弹窗关闭、服务端存储正确、scope 清空
- 重开弹窗读回 [["gpt-5.2@xinjianya"]]

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

internal/gateway/ui/index.html

@@ -832,7 +832,7 @@
           kAutoChainSave: "保存",
           kAutoChainTier: "层级",
           kAutoChainEmpty: "该密钥使用全局 AUTO 链",
-          kAutoChainBad: "槽位无法解析（模型或源不存在），保存后 AUTO 请求会失败",
+          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",
           kRoleAdmin: "管理员",
@@ -1103,7 +1103,7 @@
           kAutoChainSave: "Save",
           kAutoChainTier: "Tier",
           kAutoChainEmpty: "This key uses the global AUTO chain",
-          kAutoChainBad: "Slot does not resolve (unknown model or source); AUTO requests would fail",
+          kAutoChainBad: "Saved, but the slot does not resolve (no source serves that model); AUTO requests will fail",
           kName: "Name",
           kRole: "Role",
           kRoleAdmin: "Admin",
@@ -3694,9 +3694,17 @@ const pnote = $("#period-note");
       // different id inside its modal, so both share one implementation
       // instead of a reduced second copy that would drift.
       function sortCanvasEl() {
-        const own = document.getElementById("scr-canvas");
-        if (own) return own;
-        return document.getElementById("key-auto-canvas");
+        // The modal editor is mounted on top of the settings tab, so BOTH
+        // canvases can be in the DOM at once — #scr-canvas belongs to the
+        // hidden tab behind it. Resolve by scope, not by document order: when
+        // sortState.scope names a key we are editing THAT key's chain, and
+        // painting into the background tab's canvas both shows an empty
+        // dialog and corrupts the global chain the user comes back to.
+        if (sortState.scope) {
+          const k = document.getElementById("key-auto-canvas");
+          if (k) return k;
+        }
+        return document.getElementById("scr-canvas");
       }
       function paintSort(affected) {
             const cv = sortCanvasEl();
@@ -5283,10 +5291,12 @@ const cv = sortCanvasEl();
             body: JSON.stringify({ auto: lanesToRules(sortState.lanes) }),
           });
           if (j.inherits) {
-            toast(t("kAutoChainGlobal"));
+toast(t("kAutoChainGlobal"));
           } else if (!j.slots) {
-            // Persisted, but nothing resolves — say so now instead of letting
-            // the user meet it as a 503 on their next AUTO request.
+            // Persisted, but nothing resolves. Still close: the write
+            // SUCCEEDED, and leaving the dialog open after a successful save
+            // makes the user click Save repeatedly. The warning carries the
+            // real problem — the toast survives the modal closing.
             toast(t("kAutoChainBad"));
           } else {
             toast(t("sortSaved") + " (" + j.slots + ")");