Fix macOS packaging with Homebrew Qt on Apple Silicon

[alesaccoia]

This fixes the macOS packaging flow for Apple Silicon/Homebrew Qt installs.

What changed:

• detect Qt tools and install paths with qtpaths instead of relying on QTDIR
• copy Qt translations from the discovered Qt installation
• make mac_deploy.sh fail fast if required Qt tools or plugins are missing
• deploy only the macOS plugins FocusWriter actually needs (libqcocoa, libqmacstyle, libqjpeg)
• update the macOS install instructions to mention the Homebrew Qt workflow

Why this is needed:

• the app itself builds on Apple Silicon once Qt 6 is installed
• the old deploy script assumes QTDIR/translations exists and lets macdeployqt sweep optional Homebrew Qt plugins
• on my Apple Silicon/Homebrew setup that produced a broken DMG, with unresolved QtPdf, QtSvg, and QtVirtualKeyboard plugin dependencies during deployment

Verification:

• brew install qt
• cmake -B build -S . -DCMAKE_BUILD_TYPE=Release
• cmake --build build -j8
• run ../mac_deploy.sh from the build directory
• verified the generated DMG mounts correctly
• verified FocusWriter.app inside the DMG passes codesign --verify --deep --strict

Tested on:

• Apple Silicon Mac
• Homebrew Qt 6

Please find the resulting .dmg here for testing