🐛 add warning if webserver deps isn't installed when running dev or serve

[MFA-X-AI]

When running xaibo dev or xaibo serve without the webserver optional dependencies installed, users got a cryptic TypeError: 'NoneType' object is not callable because XaiboWebServer import failed silently. This PR adds some handling for that.

Changes:

• Added explicit None checks in dev() and serve() functions before using XaiboWebServer
• Print helpful error message with installation instructions: uv add xaibo[webserver]

Before:

TypeError: 'NoneType' object is not callable

After:

Error: XaiboWebServer is not available.
The webserver dependencies are required for 'xaibo dev'.
Install them with: uv add xaibo[webserver]

Reference:
#26