fix(mh3g): clarify accepted save path structures

[VincentAdamNemessisX]

Summary

• keep file-vs-folder support visible before opening any picker
• add an expandable user2 directory-tree example to macOS and Windows
• distinguish 3DS source, read-only current Cemu reference, and writable output contents
• document card*/quest* requirements for guild-card repair
• label macOS picker buttons as file, folder, or file/folder instead of a generic icon-only action

Path semantics shown in the UI

• exact user1/user2/user3 file is accepted
• its direct parent directory is accepted
• directory resolution is direct-only and same-slot
• ZIP/7z/RAR, recursive SD-card scans, and recursive MLC scans are not supported
• output directories resolve to /<selected user#>

Verification

• swift test — 73 passed
• python3 scripts/verify-mh3g-save-converter-windows-source.py
• python3 scripts/mh3g-docs-contract.py
• macOS app production build + synthetic smoke
• git diff --check

Runtime boundary

• macOS UI was opened and the new repair-mode labels and disclosure were inspected
• Windows native runtime remains gated by hosted Windows validation
• no emulator or real save was opened or modified

[VincentAdamNemessisX]

Lifecycle audit: this PR is not merged or superseded. Its path-selection guidance is still absent from current main, but its WinUI files conflict with the Fluent rewrite merged in #42. Keep open; rebase or re-implement the guidance on current main (preferably after #44) rather than force-merging the old XAML.