Default managed mode to transcribe.shadowdao.com and simplify login UI

- Set default server_url to https://transcribe.shadowdao.com
- Remove Server URL field from managed mode settings (users don't need to configure it)
- Replace Register button with link to website signup page
- Add fallback to default URL in login handler for existing users with empty config

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config/default_config.yaml

@@ -70,7 +70,7 @@ web_server:
 
 remote:
   mode: byok  # local | managed | byok
-  server_url: ""  # Proxy server URL for managed mode (e.g., wss://your-proxy.com)
+  server_url: "https://transcribe.shadowdao.com"  # Proxy server URL for managed mode
   auth_token: ""  # JWT stored after login (managed mode)
   byok_api_key: ""  # Deepgram API key for BYOK mode
   deepgram_model: nova-2  # Deepgram model to use