Add cross-platform development environment setup guide

docs/BUILDING.md

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+# Building OrbitHub (C++ / Qt6 Widgets)
+
+Run all commands from the repository root unless noted.
+
+## Linux (Ubuntu / Mint)
+
+```bash
+sudo apt update
+sudo apt install -y \
+  build-essential cmake ninja-build git pkg-config \
+  qt6-base-dev qt6-base-dev-tools qt6-tools-dev qt6-tools-dev-tools
+
+cmake -S . -B build -G Ninja
+cmake --build build
+./build/orbithub
+```
+
+## macOS (Homebrew)
+
+```bash
+xcode-select --install
+brew update
+brew install cmake ninja pkg-config qt@6
+
+cmake -S . -B build -G Ninja -DCMAKE_PREFIX_PATH="$(brew --prefix qt@6)"
+cmake --build build
+./build/orbithub
+```
+
+## Windows 11 (PowerShell + MSVC + vcpkg)
+
+```powershell
+winget install -e --id Git.Git
+winget install -e --id Kitware.CMake
+winget install -e --id Ninja-build.Ninja
+winget install -e --id Microsoft.VisualStudio.2022.BuildTools `
+  --override "--quiet --wait --norestart --add Microsoft.VisualStudio.Workload.VCTools"
+```
+
+Open a new terminal after installs, then:
+
+```powershell
+git clone https://github.com/microsoft/vcpkg C:\dev\vcpkg
+C:\dev\vcpkg\bootstrap-vcpkg.bat
+C:\dev\vcpkg\vcpkg.exe install qtbase:x64-windows
+
+cmake -S . -B build -G Ninja `
+  -DCMAKE_TOOLCHAIN_FILE=C:/dev/vcpkg/scripts/buildsystems/vcpkg.cmake
+cmake --build build
+.\build\orbithub.exe
+```
+
+## Notes
+
+- OrbitHub currently requires Qt6 Widgets and CMake 3.21+.
+- If Qt is installed in a custom location, pass `-DCMAKE_PREFIX_PATH=/path/to/Qt/6.x.x/<toolchain>` to CMake.