fix: normalize API URL to prevent duplicate /chat/completions path

Users entering the full endpoint URL (e.g. https://openrouter.ai/api/v1/chat/completions)
caused a 404 because the code appended /chat/completions again. Now strips any trailing
/chat/completions before re-appending, and adds a hint in the UI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/admin/AdminShadowAiPage.tsx

@@ -194,7 +194,7 @@ function ModelSlotCard({ slot, model, isLoading }: { slot: string; model?: Shado
         </Group>
         <Divider />
         <TextInput label="Display Name" placeholder="e.g. GPT-4o" value={name} onChange={(e) => setName(e.target.value)} size="sm" />
-        <TextInput label="API URL" placeholder="https://api.openai.com/v1" value={apiUrl} onChange={(e) => setApiUrl(e.target.value)} size="sm" />
+        <TextInput label="API URL" description="Base URL only — /chat/completions is added automatically" placeholder="https://openrouter.ai/api/v1" value={apiUrl} onChange={(e) => setApiUrl(e.target.value)} size="sm" />
         <PasswordInput label="API Key" placeholder="sk-..." value={apiKey} onChange={(e) => setApiKey(e.target.value)} size="sm" />
         <TextInput label="Model Name" placeholder="gpt-4o" value={modelName} onChange={(e) => setModelName(e.target.value)} size="sm" />
         <Switch label="Active" checked={isActive} onChange={(e) => setIsActive(e.currentTarget.checked)} />