fix: Always send origin header to server

The origin header was only sent when cliConfig.origin was truthy,
but we set it to null when not explicitly provided. This caused the
header to never be sent, resulting in "origin: (not set)" in Mixpanel.

Fix: Always send the header, using 'null' string when origin is null.
This ensures Mixpanel always receives the origin value.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: dannyshmueli

src/lib/api-client.ts

@@ -476,9 +476,7 @@ class SecureApiClient {
           const configHeaders: Record<string, string> = {};
           if (cliConfig) {
             configHeaders['x-vibe-config-version'] = cliConfig.version;
-            if (cliConfig.origin) {
-              configHeaders['x-vibe-config-origin'] = cliConfig.origin;
-            }
+            configHeaders['x-vibe-config-origin'] = cliConfig.origin || 'null';
             configHeaders['x-vibe-config-statusline'] = JSON.stringify(cliConfig.statusline);
             configHeaders['x-vibe-config-hooks'] = JSON.stringify(cliConfig.hooks);
           }