fix(webui): env vars editor loses focus on every keystroke

The environment variable key was used as the React key prop, so
editing the key name caused React to unmount/remount the input
on every character typed.

Switched to array index as key — stable across edits.

Confidence: high
Scope-risk: narrow
Tested: manual verification, tsc clean

Author: Helmi

client/src/components/AgentConfigEditor.tsx

@@ -548,8 +548,8 @@ function EnvVarsEditor({ env, onChange }: EnvVarsEditorProps) {
 
   return (
     <div className="mt-2 space-y-2">
-      {entries.map(([key, value]) => (
-        <div key={key} className="flex items-center gap-2">
+      {entries.map(([key, value], index) => (
+        <div key={index} className="flex items-center gap-2">
           <Input
             value={key}
             onChange={(e) => updateEntry(key, e.target.value, value)}