fix: detect and display env var API config fallback (closes #22, closes #4)

Providers API now detects ANTHROPIC_API_KEY, ANTHROPIC_AUTH_TOKEN, and
ANTHROPIC_BASE_URL from environment. ProviderManager shows a banner
when env vars are detected, informing users they're used automatically
when no provider is active.

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

src/app/api/providers/route.ts

@@ -1,6 +1,6 @@
 import { NextRequest, NextResponse } from 'next/server';
 import { getAllProviders, createProvider } from '@/lib/db';
-import type { ProvidersResponse, ProviderResponse, ErrorResponse, CreateProviderRequest, ApiProvider } from '@/types';
+import type { ProviderResponse, ErrorResponse, CreateProviderRequest, ApiProvider } from '@/types';
 
 function maskApiKey(provider: ApiProvider): ApiProvider {
   let maskedKey = provider.api_key;
@@ -10,10 +10,35 @@ function maskApiKey(provider: ApiProvider): ApiProvider {
   return { ...provider, api_key: maskedKey };
 }
 
+/** Check which ANTHROPIC_* env vars are set in the server process environment */
+function detectEnvVars(): Record<string, string> {
+  const detected: Record<string, string> = {};
+  const envKeys = [
+    'ANTHROPIC_API_KEY',
+    'ANTHROPIC_AUTH_TOKEN',
+    'ANTHROPIC_BASE_URL',
+  ];
+  for (const key of envKeys) {
+    const val = process.env[key];
+    if (val) {
+      // Mask secrets, show base_url in full
+      if (key.includes('URL')) {
+        detected[key] = val;
+      } else if (val.length > 8) {
+        detected[key] = '***' + val.slice(-8);
+      } else {
+        detected[key] = '***';
+      }
+    }
+  }
+  return detected;
+}
+
 export async function GET() {
   try {
     const providers = getAllProviders().map(maskApiKey);
-    return NextResponse.json<ProvidersResponse>({ providers });
+    const envDetected = detectEnvVars();
+    return NextResponse.json({ providers, env_detected: envDetected });
   } catch (error) {
     return NextResponse.json<ErrorResponse>(
       { error: error instanceof Error ? error.message : 'Failed to get providers' },