The first thing we'll need is a way to change mouse coordinates into what cell they correspond to:

void MouseToRowCol(int32 x, int32 y, uint32& r, uint32& c)
{
    if (x < 0 || x > WINDOW_WIDTH || y < 0 || y > WINDOW_HEIGHT)
    {
        x = 0;
        y = 0;
    }
    
    r = std::floor(float(y) / float(IMAGE_HEIGHT));
    c = std::floor(float(x) / float(IMAGE_WIDTH));
}

Cell* MouseToCell(int32 x, int32 y)
{
    uint32 r;
    uint32 c;
    MouseToRowCol(x, y, r, c);
    return &g_cells[r][c];
}
        

Next, we can deal with SDL_Event's like so:

if (e.type == SDL_MOUSEBUTTONDOWN || e.type == SDL_MOUSEBUTTONUP)
{
    if (lost && e.type == SDL_MOUSEBUTTONUP)
    {
        InitCells();
        lost = false;
        while (SDL_PollEvent(&e) != 0) {}
        break;
    }

    uint32 r;
    uint32 c;
    MouseToRowCol(e.button.x, e.button.y, r, c);
    Cell* cell = MouseToCell(e.button.x, e.button.y);

    if (e.type == SDL_MOUSEBUTTONUP)
    {
        if (e.button.button == SDL_BUTTON_MIDDLE)
        {
            // Reveal cells
        }
        else if (e.button.button == SDL_BUTTON_LEFT)
        {
            // Click a cell
        }
    }
    else if (e.type == SDL_MOUSEBUTTONDOWN && e.button.button == SDL_BUTTON_RIGHT)
    {
        // Flag or guess
    }
}