fix: remove API_SECRET from .env template to restore web UI login

When API_SECRET is set, Invoice Ninja requires the X-API-SECRET header
on ALL API calls including /api/v1/login. The web UI (React SPA) never
sends this header, so browser login silently fails with "Invalid secret"
while the mobile app (which can be configured with the secret) continues
to work.

UPDATE_SECRET serves a different purpose (protecting the /update endpoint)
and is kept. API_SECRET should remain unset for self-hosted YunoHost installs.

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

conf/default.env

@@ -57,7 +57,8 @@ PHANTOMJS_KEY='__PHANTOMJS_KEY__'
 PHANTOMJS_SECRET=__APP_KEY__
 
 UPDATE_SECRET=__API_SECRET__
-API_SECRET=__API_SECRET__
+# API_SECRET is intentionally not set: when set, it requires the X-API-SECRET header
+# on ALL API calls including the web UI login (React SPA), which breaks browser login.
 
 PRECONFIGURED_INSTALL=true