fix fvm global env and per project

confusion about path sdk using FVM, about global env, and per project:
https://github.com/leoafarias/fvm/issues/599

will fix with this:

export FLUTTER_HOME="$HOME/fvm/default"

proxy_flutter() {
    local flutter_exec
    if [[ -x "./.fvm/flutter_sdk/bin/flutter" ]]; then
        flutter_exec="./.fvm/flutter_sdk/bin/flutter"
    elif [[ -x "$HOME/fvm/default/bin/flutter" ]]; then
        flutter_exec="$HOME/fvm/default/bin/flutter"
    else
        echo "Flutter executable not found. Make sure it's installed and paths are correctly set."
        return 1
    fi

    "$flutter_exec" "$@"
}

# Alias flutter command to our proxy method
alias flutter='proxy_flutter'

proxy_dart() {
    local dart_exec
    if [[ -x "./.fvm/flutter_sdk/bin/dart" ]]; then
        dart_exec="./.fvm/flutter_sdk/bin/dart"
    elif [[ -x "$HOME/fvm/default/bin/dart" ]]; then
        dart_exec="$HOME/fvm/default/bin/dart"
    else
        echo "Dart executable not found. Make sure it's installed and paths are correctly set."
        return 1
    fi

    "$dart_exec" "$@"
}

# Alias Dart command to our proxy method
alias dart='proxy_dart'

vscode setting:
dart.addSdkToTerminalPath = true / ✅

this make flutter doctor -v all green