feat: system-wide mango commands, folder browser, and test suite

[tmarhguy]

Summary

• Install all Mango tools globally via ./setup.sh --link --yes (exec wrappers in ~/.local/bin)
• Fix launcher symlink resolution so mango works from any directory without hanging
• Add folder browser for file selection — only shows subfolders that contain matching files
• Fix CLI flag parsing (-a for hash-gen, -p for protect/unlock PDF) and base64 roundtrip
• Add layered test suite (ShellCheck, Ruff, pytest, browse tests, smoke) and GitHub Actions CI
• Update docs and screenshots for the new UI flow

Test plan

• ./setup.sh --link --yes then mango from a directory with nested image files
• Browse into a subfolder, select a file, run Compress Image end-to-end
• ./tests/run.sh --quick
• ./tests/run.sh (full smoke; video tests need ffmpeg)