Fix dev prerequisite guidance

[Finesssee]

Summary

Fixes dev.ps1 so missing Rust/MinGW prerequisites no longer try to run the deleted scripts\setup-windows.ps1 bootstrap script. The script now exits with current prerequisite guidance for Rust/Cargo, Node/pnpm, Visual Studio Build Tools, and MinGW only when the shell is using a GNU Rust target.

Also updates the stale source-build doc to describe the current MSVC-based Windows prerequisites instead of the removed setup script.

Closes #92

Affected areas

• Tray panel
• Settings UI
• Config file / settings persistence
• CLI
• Provider-specific behavior
• Installer / release packaging
• Startup / background behavior
• Documentation
• Other:

Validation

• cargo test --manifest-path rust\Cargo.toml
• cargo test --manifest-path apps\desktop-tauri\src-tauri\Cargo.toml
• cargo fmt --all
• cargo clippy --all-targets -- -D warnings
• pnpm --dir apps\desktop-tauri test
• pnpm --dir apps\desktop-tauri run build
• Thermo-nuclear code quality review completed before submitting: https://github.com/cursor/plugins/blob/main/cursor-team-kit/skills/thermo-nuclear-code-quality-review/SKILL.md
• Parsed dev.ps1 with PowerShell parser
• git diff --check -- dev.ps1 extra-docs\BUILDING.md
• Verified no remaining setup-windows.ps1 / Setup script not found references in dev.ps1, docs, or READMEs

UI / tray proof

• Not applicable
• CUA Driver visual proof attached
• CUA Driver could not be used; equivalent manual proof and explanation attached

Notes for reviewers

This is a script/docs fix for a removed bootstrap file, not a runtime app behavior change.