fix: provider-aware error messages and skip Anthropic key approval for 3P

1. errors.ts: Add getCustomOffSwitchMessage() that returns a
   provider-neutral message for 3P users instead of the hardcoded
   "Opus is experiencing high load, please use /model to switch to
   Sonnet" which is misleading for OpenAI/Gemini/Ollama users.
   The original constant is preserved for backward-compatible string
   matching in error handlers.

2. Onboarding.tsx: Skip the "approve API key" step when a 3P provider
   is active. Previously, having ANTHROPIC_API_KEY in the environment
   (e.g., from a previous Anthropic setup) triggered an irrelevant
   Anthropic key approval UI even when using Gemini or OpenAI.

src/components/Onboarding.tsx

@@ -99,7 +99,7 @@ export function Onboarding({
     // Add API key step if needed
     // On homespace, ANTHROPIC_API_KEY is preserved in process.env for child
     // processes but ignored by Claude Code itself (see auth.ts).
-    if (!process.env.ANTHROPIC_API_KEY || isRunningOnHomespace()) {
+    if (!process.env.ANTHROPIC_API_KEY || isRunningOnHomespace() || !isAnthropicAuthEnabled()) {
       return '';
     }
     const customApiKeyTruncated = normalizeApiKeyForConfig(process.env.ANTHROPIC_API_KEY);