world-mutation: route server_frame Join through tool_work_world_t

[janry]

The multiplayer-browser Join handler was the one remaining call site
calling welt->load() synchronously from a GUI event handler.  Fire
TOOL_WORK_WORLD with default_param="l0net:<server>" instead, matching
the pattern loadsave_frame already uses; the load runs from command_queue
once the click handler has unwound.  tool_work_world_t's 'l' branch now
distinguishes net: from local filenames to set CLIENT_WORLD vs LOADED_WORLD
post-load.

[prissi]

Thanks, incorporated.