fix: auto-reset web session on new client chat & make cookie optional with fallback

[zenyxx-xd]

Summary of Changes

1. Auto-reset Web Session on New Client Conversation:

   • Problem: When agent clients like OpenCode or Claude Code connect locally without sending x-agent-session, agentId defaults to 'dev-agent'. When a user starts a fresh conversation in OpenCode, FreeDeepseekAPI was reusing the previous chat_session.id on DeepSeek Web, mixing messages from old sessions into the same Web chat.
   • Fix: Added a check in server.js: if an incoming payload contains zero assistant responses (messages.some(m => m.role === 'assistant') === false), FreeDeepseekAPI recognizes that the client started a brand new conversation and calls resetRemoteSession(session) so DeepSeek Web creates a fresh chat_session.id.

2. Make cookie Optional with Default Fallback:

   • Problem: When users input an auth token without pasting full browser cookies, a.config.cookie is empty (""). server.js was rejecting accounts with No valid DeepSeek auth accounts due to strict a.config.token && a.config.cookie filters.
   • Fix: Added a default fallback header ("Cookie": config.cookie || "ds_cookie_preference=%7B%22level%22%3A%22all%22%7D;") and relaxed the strict a.config.cookie requirement in server.js & scripts/doctor.js, allowing token-only auth to work out-of-the-box.