Rewrite User Guide to match docs; add interface diagram; fix File Reference
User-Guide: full rewrite to match docs/user-guide structure — adds First
Launch, Search/Filter, Script Details, Script Notes, Run with Arguments,
Sorting, Update Dependencies, Themes, GitHub Token, In-App Help, Writing
Scripts, and interface diagram.
File-Reference: add missing qbar_target_exe field to Settings struct.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>